It’s frowned upon in America to wish someone a ‘merry Christmas’. We were there one Christmas a couple of years ago and the best wishes were a simple and boring ‘happy holidays’. ‘Merry Christmas’ is an absolute no-no.
It’s politically incorrect. Someone, somewhere, might be offended.
The old saying of ‘when in Rome do as the Romans do’ was ignored in our case, but no-one seemed to care. We were Australians, so perhaps we were excused for our ignorance.