Paul also says that when something is done (or in this case, not done) that causes a brother to be offended, we sin against that brother.
So where do we draw the line?
He was talking about a weaker brother being offended.
Those of us who sit out there and snipe at these churches are not in danger of losing our faith over their less than stellar decision.
What I would do is lovingly let them know that they've made a weak decision.
The penalty to them is that none of the Christmas attenders will go to their churches.