I am Jewish, was born Jewish, (although I am not practicing) and I have NEVER had a problem with people saying "Merry Christmas." This country is majority Christian, that is a FACT, and anyone who gets all up in arms over Christmas decorations, Christmas greetings, Christmas sales at stores, or whatever, is a whining politically-correct jerk.
For the record, I have never met a single black person who actually celebrated Kwanzaa. Sometimes I suspect that the whole holiday was completely made up as a big prank on America, and that it doesn't actually exist.