I mean it's a just fucking decoration. If people want to get that legalistic about it, then fine I guess. But it's not a huge deal. It's certainly not worth anyone's time and it will not harm our country in any way whatsoever. The only reason anyone even knows about it is because this guy wants money. If this guy really hates Christianity that much then maybe he should just stop being an intolerant fuck.
They could put a Jewish star on top of the tree, or some Atheist symbol or whatever. Who cares?