Sechel, that’s not how psak works.
Jews in America are not beholden to poskim elsewhere; American poskim allowed it because rav moshe allowed it. It became accepted as the minhag haolam, and that is perfectly fine when someone who is accepted as the gadol hador issues a psak and is not challenged by many other gedolim in his country.
Most American jews accepted rav moshe as their rebbe in general, and there are very few psakim from rav moshe that aren’t followed, such as music, yerushos, and Shabbos timers (that’s about the extent of the list)