Why is המחזיר אבידה לכותי singled out?

Question

The Gemara, Sanhedrin 76b, states that "one who returns a lost object to a כותי (or, per the marginal notes there, גוי) is censured by the Torah. Rashi there explains: "He is thereby equating the כותי with the Jew, and demonstrates that he does not consider returning a lost object to be the subject of his Creator's commandment, since he is returning it even to a כותי, about whom he wasn't commanded."

Now, whether this rule applies nowadays, and to whom, is a whole other issue which I'm not getting into right now. My question is simply this: We know that there are other Torah laws in which similar distinctions are drawn - for example, the mitzvah to lend to a Jew without interest (as contrasted with a non-Jew), or the rules of לא מורידין ולא מעלין (Avodah Zarah 26a-b and from there in Choshen Mishpat 425:5). So why does the Gemara single out "returning a lost object" more than any of these?

Answer 1

Returning a lost object to an idol worshiper is seen as strengthening his hand. According to law you can keep it, yet you go out of your way to give it to him....

Answer 2

One could do the other actions for a side reason:

1. One could lend money to a non-Jew without interest to be on his good side (and the non-Jew may give him a favor back later on).
2. One may want to save a non-Jew's life as he may be able to save a Jew's life.

Moreover, one keeping those two carefully might result in "Eiva". For example,

1. Jews were always known (by the Anti-Semitic masses) as being "greedy Jews" who lend money with interest and then take as payment everything they own. To avoid this "repution", ne may want to lend money without interest. ([R' Yosel of Rosheim, for example, forbade charging "too much interest" to non-Jews as a compromise with the Holy Roman Empire).

2. If a Jew was caught not saving a non-Jew's life, he may be accused of murder, etc.

However, if a Jew doesn't return a lost object, no one would really be able to accuse him as:

1. In the non-Jewish world, "finder's keepers" is a fully accepted principle.
2. It's unlikely that people would know that he found an object and didn't return it.

Answer 3

[I believe some of the cases mentioned have technicalities. The gemara in Avoda Zara seems? to apply specifically to idol worshippers, not gentiles in general. Medical treatment on shabbos is a desecration of Shabbos without the permit that applies to Jews (so they will keep future Shabbosos). We don't need a special pasuk to tell us not to violate Shaboss]

Hashovas Aveida has no specific forbidden act, and yet is different from other chesed. G-d wants you to emulate Him by doing chesed. Chesed may apply more so to fellow Jews, but isn't exclusive. Hashovas Aveida, on the other hand, is not a chesed. If lack of possession by the original owner undermines ownership, there is no chesed to give someone something that does not belong to him. The obligation to return a lost object to a Jew is not because we do bigger chesed, it is because we have a different system for losing ownership. So if I return an lost object to a gentile using the same system, that's not chesed. It's applying a system to the wrong set and demonstrates a lack of appreciation for G-d's commandments.

[Interest is permitted as a chesed to gentiles at least under the category of ger toshav.]