פירוש על עבודה זרה 45:10

The Talmud now rejects Shila based on the continuation of the mishnah. R. Eliezer does not allow one to buy any animals for sacrifice from non-Jews. Yet only with regard to the red heifer does the verse state explicitly that only an Israelite must bring it. Therefore, R. Eliezer’s reading cannot be based on that midrash. It must be that he is suspicious about bestiality.

The Talmud now tries another tack. Maybe the rabbis too are concerned about bestiality. But they assume that a non-Jew will not have sex with the red heifer because that would be a great loss. But when it comes to other sacrifices, maybe they would have sex with the animal. After all, it’s not such a big financial loss if the Jew won’t buy it.