"Why do so some Christians find it offensive to welcome other groups as well"
It has nothing to do with "welcoming other groups". It is the cleansing of Christianity completely from our society. They are kowtowing to litigious athiests. No religious group that I've ever heard of had any problem with Christmas, it's the Newdows of this country bitching and filing lawsuits. Atheism is a religion, and is fast becoming our state religion
I was raised in a very conservative Church of Christ, and they did not observe Christmas. We had a Christmas tree at home and gave presents, but I was always taught that the celebration of Christmas was one instituted by man, not God, and there was no observance at church. I know there are other churches that don't observe Christmas either. So, yes there are religous groups that have a problem with Christmas, they just don't make a lot of noise about it.
But yet these same people who complain have not the courage to say a word to the Muslims! Why is that?