As discussed in the other thread, many Christians, non-Christians, and non-religious people alike celebrate Christmas and say "Merry Christmas." I work with people of different national backgrounds, ethnicities, and faiths, and many of them exchange presents, listen to Christmas music, and greet people "Merry Christmas" during the holidays.
I think what pisses off a lot of religious people (apart from the commercialism) is the attempt at "secularization" of Christmas, mostly by the opponents on the other side, as I mentioned in the other thread, the uber-PC. The uber-PC say things like "Christmas is so ... Christian. Why can't we call it something else or celebrate something else?" One cannot deny the fact that Christmas was initially conceived by the Church as a religious holiday to celebrate the birth of Jesus Christ, so what is really being celebrated on December 25? Christians have as much right to observe it in a religious way as much as the Jews celebrate Hanukkah or Africans celebrate Kwanzaa. On the other hand, Christmas is an amalgation of both Christian
rituals and traditions, so I can see why it's got such a universal appeal and some non-Christians prefer to celebrate it as well.