I'm Jewish and I'm certainly not offended when people wish me a Merry Christmas any more than I am offended if they mispronounce my name. It just shows the person doesn't know me. But I take no offense. I never correct strangers who wish me a Merry Christmas. They mean well. Some of my wife's relatives used to wish me Merry Christmas (she and her family are Christian). I thanked them for their well-wishes and gently explained to them that I don't celebrate Christmas. I did this not to make a PC statement or embarass them, but because I just wanted them to know me better.
Part of being an American and living here is understanding that this country is mostly Christian. Nonetheless, I find it odd that someone who does not know me would wish me a Merry Christman, which assumes I celebrate Christmas. The population of the U.S. is about 1% Jewish and about 3% Bhuddist, Muslim and other non-Christian religions. It is also about 15-20% agnostic/atheist. So there is a reasonable chance that your average American does not celebrate Christmas, at least as a religious holiday.