The gemara says that even though the prohibition of אבר מן החי for a Jew may only apply to kosher animals (acc. to chachamim and R. Meir), for a goy it will apply even to non-kosher animals. Tosafos 33a questions how this can be. Wouldn't this be a violation of the rule that there is nothing prohibited to a goy that is permitted to a Jew - ליכא מדעם דלישראל שרי ולעכו"ם אסור. Tosafos says that since there would be a prohibition for the Jew to eat a non-kosher animal, even though it wouldn't be considered אבר מן החי, it would not violate the principle of something being permitted to a Jew and prohibited to a goy. Tosafos holds that the prohibition on the goy doesn't have to be the same prohibition on the Jew so long as there is some prohibition (R. Akiva Eiger in a teshuva 165 qualifies this idea and says that it would only be when the prohibition on the Jew is a lack of shechita such as a non-kosher animal, to the exclusion of a treifa where it has already been shechted properly and has not relation to the issur of eiver min ha'chai).
It seems to me that Rashi in our sugya is also addressing the same question. Rashi writes -
אבל בן נח מוזהר על הכל, דכל דקרינא ביה בשר לחודיה אכול, קרינא ביה אבר מן החי לא תאכל. Rashi says that whenever it is permitted to eat the meat after being killed, there is an issur of eiver min ha'chai to eat the meat prematurely. Perhaps rashi understands that the principle of there is nothing prohibited to a goy that is permitted to a Jew isn't violated here because the same exact principle that applies to the goy - don't eat alive what you can eat dead, applies to the Jew. The only difference is that the Jew cannot eat non-kosher animals when dead so there is not issur of אבר מן החי when it's alive, but for the goy there is. Since the principle applies equally to the Jew and goy it wouldn't violate the rule of ליכא מדעם דלישראל שרי ולעכו"ם אסור.