Why does the Rabbi think there is no problem using utensils that have not been immersed and belong to a Jew?
(Seemingly, even according to the medieval authorities who hold that use is permitted even before immersion, the later authorities wrote [see Biur Halakha, sec. 323:7] that rabbinically, at any rate, it is prohibited.)

Because regarding a restaurant, halakhic decisors wrote that one may be lenient for two reasons: the utensils were not acquired for eating but for making profit (a consideration that seems doubtful to me). The obligation to immerse rests on the owner of the utensils, and therefore the prohibition on eating as well—which is a penalty—was imposed on the one whose obligation it is to immerse them. On the basis of the second reason, some decisors are lenient even for a guest in a private home.

The first consideration is novel in my eyes too (which is why I asked).
The second consideration is even more novel (although I seem to remember that the Bach wrote this—maybe the Rabbi remembers a more precise source), and not only according to Maimonides (as several decisors understood him), where immersion is a kind of permit for using the utensil; even according to the other decisors, the straightforward understanding is that this is a kind of rabbinic prohibition and not a penalty.
Another consideration I saw for permitting this (in the name of Rabbi Shlomo Zalman Auerbach) is that the obligation of immersion is like the obligation of the commandment of tzitzit according to the Mordechai, and therefore if a person is under compulsion, there is no problem with drinking from the utensil without immersing it first (and Rabbi Shlomo Zalman defines it that way regarding all rabbinic commandments). But aside from the fact that the definition of compulsion here still requires investigation, the claim itself is also novel.
And since according to several medieval authorities the obligation of immersion is Torah-level, shouldn’t one be stringent about this?
(As a new immigrant, this was a new stringency for me that wasn’t really relevant abroad, since there it’s easier to drink in a café owned by a non-Jew.)

By the way, I only just noticed that the original questioner is also named Yishai, so sorry for the confusion 🙂

The Rabbi wrote: “Regarding a cup of coffee, it is worth checking that it was not washed together with meat utensils.”
Does washing together with meat utensils render the vessel non-kosher?
After all, even if they wash the dishes in warm water, it doesn’t reach a temperature that transfers flavors at all, and in addition they use dish soap, which spoils the taste of any food residue.

Indeed, it depends on the details of the washing and the material the utensils are made of. I wrote in general terms.

Second Yishai, I don’t know what the difference is between a rabbinic prohibition and a penalty. If you mean the practical difference I mentioned (whether it is imposed on the owner or not), then I wrote my view. The formulation about tzitzit is probably a “permitting argument,” but regarding tzitzit that is simply not correct, and this isn’t the place for it.

I wasn’t understood correctly because I tried to be brief.
There is a discussion among the medieval authorities whether utensils that were not immersed are permitted for use. Beyond that, there is a discussion, according to those who prohibit using them (as seems to be the view of Maimonides), whether immersion is a permit or a commandment (with a practical difference regarding “fixing a utensil”; see Sha’agat Aryeh, no. 56).
The later authorities (I already pointed you to Biur Halakha) wrote that even according to the medieval authorities who permitted it, the utensil is still rabbinically forbidden for use.
And even here one can investigate as above, but none of this makes a practical difference for our case, because according to this it does not matter who owns the utensils.
The Rabbi gave a novel interpretation of this prohibition, namely that it is only a kind of penalty, and therefore it applies only to the one who owns the utensils. My point was that this runs against the straightforward understanding of the later authorities; and on what basis should we assume that the Sages imposed a penalty rather than assume that it is a kind of parallel to its Torah-level counterpart?
Besides that, I brought another reason to permit it in the name of Rabbi Shlomo Zalman—that rabbinic prohibitions resemble the law of tzitzit that tore on the Sabbath according to the Mordechai (not that tzitzit are a permit for a four-cornered garment, but that the prohibition stems from the commandment), and therefore if he cannot immerse it (for example, because it is not his), he may use it. And about that too I commented as above.
By the way, does the Rabbi agree with my understanding of the Mordechai?

*With (in the second-to-last line)

For an overview and sources, see for example here:
https://www.yeshiva.org.il/midrash/7172

“Fixing a utensil” does not necessarily mean there is a prohibition for others. Even fixing a utensil for one particular person is a prohibition. For example, separating terumah is a prohibition even according to the views that allow priests to eat untithed produce (see Atvun DeOraita, which investigates whether the prohibition of untithed produce is because of the terumah mixed into it, and cites in this a dispute among Rashi, Tosafot, and other medieval authorities).

If I understood correctly, what you wrote—that the prohibition stems from the commandment—is just a roundabout way of talking about neglecting a positive commandment. Do you mean to claim that eating with utensils that have not been immersed involves neglect of the positive commandment of immersion? That is parallel to the penalty approach (though of course not identical to it). If we are dealing with a rabbinic prohibition (and I think that is the accepted view), then obviously this is not neglect of a positive commandment, and it is very plausible that it is a penalty.
And regarding the Mordechai’s permission, one could analyze it at length, but this isn’t the place. Offhand, from there it דווקא seems that this is a penalty and not neglect of a positive commandment.

I don’t want to burden the Rabbi, so I’ll just clarify my words and return to the practical question.
Regarding “fixing a utensil,” I meant that Sha’agat Aryeh discusses whether immersing utensils on the Sabbath involves “fixing a utensil,” and on that I said that there is a practical difference whether it is a permit or a commandment (and the later authorities already wrote this, though of course it is not conclusive).
As for Rabbi Shlomo Zalman’s argument, that is more or less what I meant, but what I mainly wanted to clarify is whether the Rabbi actually rules this way in practice despite the words of most later authorities, since it does not seem so from what they write, and in my opinion that is also the straightforward meaning of the medieval authorities (aside from Maimonides).
(I will bring here only the wording of Pri Megadim: “It is a scriptural decree that the Merciful One prohibited using a utensil without immersion.”)
Does the Rabbi think this way regarding all rabbinic prohibitions that come in connection with positive commandments?
As for the Mordechai, I didn’t understand—there we really are dealing with neglect of a Torah-level positive commandment, the commandment of tzitzit, so what exactly is the Mordechai discussing as the definition of the positive commandment, and where does a penalty come in? (By the way, Sha’agat Aryeh in no. 32 argues that there is a contradiction in Rabbi Isaac’s opinion on this matter, but this isn’t the place.)
Thank you very much for the detailed response.

I understood the Sha’agat Aryeh (I’m familiar with it), and what I wrote about it still stands.
I think so in practice as well, if only because that is the reasonable conduct nowadays.
I haven’t checked. Every prohibition has to be examined on its own merits. There is also a difference between a prohibition derived from a positive commandment and a regular positive commandment, of course.
The Mordechai writes that it is permissible to wear tzitzit that tore on the Sabbath when there is no way to fulfill the commandment. If this is only a matter of neglecting a positive commandment, I do not see any reason to permit it in such a situation. Therefore it actually makes more sense to see the view of those who prohibit it as a penalty, against which the Mordechai argues that in such a case they did not impose a penalty. But I don’t have it in front of me now.

Ah, I thought you were coming to contradict what I said (regarding the Sha’agat Aryeh).
As for the Mordechai, in the passage there it doesn’t seem so to me, but I want to stay within the rule of “one who asks relevantly” (see Rabbi Ovadiah of Bartenura on Avot 5:7), so I won’t go into it 😉
I have a bit of a hard time with bringing considerations of reasonableness into Jewish law from the outside, rather than internally from within the Talmudic passage, but by this point I’ve really wandered off the topic.
Thanks