Question

The Talmud in Kiddushin 22 brings a baraita that expounds the verses about a Hebrew slave who wants to have his ear pierced (in Exodus and Deuteronomy), and derives from them a list of restrictions such that only if they are met is he permitted to have his ear pierced. Ibn Ezra on Exodus writes about this: “And the transmitters of the Torah said that a Hebrew slave is not to have his ear pierced if even one of the conditions is lacking, such as love of his master and his household and his wife and his children, and that it be good for him with him. And so too with the stubborn and rebellious son. And their words are correct.” Apparently the alternative, were it not for what the transmitters of the Torah said, would be to interpret this as only an example, and the same law would apply in any situation where the slave wants to have his ear pierced of his own free will—that is, like an internal paradigm case within the topic of piercing the ear of a Hebrew slave.
In the case of the stubborn and rebellious son, it seems fairly clear that the reasoning guiding the expounders is that the plain meaning sounds far-fetched: if a son does not listen to his parents’ voice and is a glutton and a drunkard, then all the men of his city should stone him to death, and so you shall remove the evil from your midst, and all Israel shall hear and fear. And because that reasoning is so strong, even weak derivations are enough for it (and the weaker the reasoning, the stronger the minimal derivation needs to be). But what led the expounders, in the case of the Hebrew slave, to go beyond their proper bounds—granted, not to the same degree as with the stubborn and rebellious son, but still the effort and the tendency are noticeable—in order not to allow a Hebrew slave to have his ear pierced?