In the former case we have primarily protestant Christians and in the latter case primarily atheists (joined by Jews in some places), but in both cases the casus belli is the same: I don't believe what you do, so I don't want you to enjoy what you do--and I certainly don't want to see you enjoy it. Throw in a religious component and every killjoy in the country thinks they can cloak themselves in the mantle of the First Amendment.
This is why they're wrong: while Christmas, and Easter, and Halloween, and Thanksgiving, all have a religious component, they also have a secular component. Secular society has co-opted those holidays and created new components of them completely divorced from the religious side. There's nothing at all religious about a fat man living at the North Pole or snowflakes or bunnies' delivering chocolate eggs or wearing costumes whilst begging for candy or watching football after eating Turkey. There just isn't, and to complain about these things year after year is a silly waste of time that shows just how soft--and how soft-headed--our society has become. There's no legitimate argument against having a Christmas tree in a classroom, or for that matter any other decoration that excludes overtly religious symbolism.
What started me down this particular road on this beautiful Sunday morning? This post about Halloween from Joanne's site. I like her conclusion, though:
Parents who think Halloween is the work of the devil must be frustrated by how much fun the holiday is for its celebrants. It’s tough to compete.