A heretic is punished, so my question was why he is punished. So even if it is a description and not an insult, that does not solve the problem.
One who says, “What benefit have the rabbis given us?” is only an example of a heretical view.
I did not understand #1—toward what is he supposed to make an effort? Toward what his neighbor on the bench thinks?!

Where is the heretic punished?

He is supposed to make an effort to clarify the truth.

If there is a commandment to believe, then automatically one who does not believe / is a heretic is punished. Maimonides also wrote the Thirteen Principles in the Mishneh Torah (an anecdote: in one of Rabbi Moshe Shapira’s lectures he was asked why Maimonides did not write his Thirteen Principles in the Mishneh Torah if he held they were so fundamental. And he answered with his usual sharpness that this is a good question, and apparently because of it Maimonides eventually did put it into the Mishneh Torah…). I do not remember right now where, or whether it is formulated as a command. What I do have in front of me now are his words at the beginning of the Laws of the Foundations of the Torah, that “knowledge of this matter—that there is a Being who exists, and He brings all into existence, etc.—is a positive commandment, etc.”

First of all, the very idea of a commandment to believe is itself quite puzzling, and he has already been thoroughly attacked over this, and several answers have been given. So no proof can be brought from here for anything. On the contrary, these difficulties show that it is not really possible to command belief, and in any case it is also not possible to punish someone who does not believe.
By way of analogy: is someone who did not have a son or daughter punished for neglecting the positive commandment of being fruitful and multiplying? Certainly not. However, if he did not make the effort, he is punished. The same applies to the commandment of faith/belief (even assuming there is such a commandment).
As best I recall, the Thirteen Principles are not really written explicitly in the Mishneh Torah. Some of them appear in one form or another in the Laws of the Foundations of the Torah, and a bit more in the Laws of Repentance.

Laws of Repentance 3:6: “These are those who have no share in the World to Come, but are cut off, destroyed, and judged for the magnitude of their wickedness and sin forever and ever: the minim, the apikorsim, those who deny the Torah, those who deny the resurrection of the dead, and those who deny the coming of the redeemer…” In halakhot 7 and 8 he includes five among the minim, three among the apikorsim, and three among those who deny the Torah (all in line with the order of the principles); together with one who denies the Torah and the resurrection of the dead, that makes thirteen.