LOLIt's a matter of identity. It's the same reason the frequent Holocaust denial promoted by the right these days is deeply offensive (and something the left had a fair share of while Bush was in the White House, to be fair).
WHAT?!?!?!?!?!
so, should Christians be offended if a non-Christian celebrates Christmas?
In the commercial sense? No.
If they started claiming it as a holy day for themselves? Yes!
this shows how little you really understand