Ever say "Merry Christmas" to someone who then responds "I'm Jewish" or "I'm Hindu"? You feel kinda stupid. It's presumptuous to assume that someone celebrates a specific holiday, although the odds are pretty high in the US that the person celebrates Christmas, and most people aren't big enough jerks to get mad at someone for wishing them a Merry Christmas.
As far as non-Christians celebrating Christmas, Christmas was a pagan holiday (Weinacht, the winter soltice, or whatever) that was coopted by the Church to make the pagan day into a Christian day. They just pulled the "Jesus was born on Dec 25th part" out of the air so it would happen on the same day every year (learning from the mistake of the Jews, perhaps), and dates from 2000 years ago don't line up well anyway.
