I have to admit, I think this thread is built on a faulty premise. I don't think overall there are a lot of people who are "offended" by "merry christmas," and that is reflected in this thread. The real problem that arises is with the people who are offended when someone says "Happy Holidays" instead of "merry christmas."
Stores say happy holidays not because they are worried that people will be offended by merry christmas, but because they recognize that they have customers who don't celebrate christmas, and they want to include those customers in their activities.
Compare it to walking down the street. You could just walk on, keeping to yourself. Fine, no one is going to be offended by that. However, just to be friendly, you smile and say hi to the people you meet. Would Bill OReilly complain about that? Is that "anti-people who keep to themselves when walking"?
In the end, the question to me isn't "Why is anyone offended when someone says merry christmas?" but instead, "Why is anyone offended when I chose to say happy holidays instead?" Because I think there are a lot more of the latter than the former.