Tosfos writes that this rule does not apply to something which -- to the contrary -- is a *Mitzvah* for a Jew, like Shabbos. A Nochri is forbidden from keeping Shabbos, while a Jew has a Mitzvah to keep Shabbos; obviously, there cannot be such a prohibition for a Jew like there is for a Nochri. Tosfos says that this will also answer why a Jew may kill a fetus to save the mother (even if we say that a Nochri may not do so) -- for it is a Mitzvah to save the mother.

In general, the rule is only that it must also be prohibited for the Jew. However, the severity of the prohibition need not be the same; there are transgressions for which a Nochri is Chayav Misah while they are only prohibited (with no Chiyuv Misah) for a Jew.

D. Zupnik