"Merry Christmas" seems to say "get drunk for Jesus". So I fully understand why Christians are offended. It is obvious why they would prefer "happy holy days". [/sarcasm]
(I did check an online dictionary. It seems that I am going by the British meaning of "merry" rather than the American meaning. I grew up down under, where British meanings are commonly used unless there is a distinctive oz meaning.)
Fundamentalism - the anti-American, anti-Christian branch of American Christianity