Because some atheists are self-important jerks. Just as some Christians are, and some Buddhists, some Jews, etc.
I believe, however—and I could be wrong—that the reason some rage against the machine is that they hate God and love their sin
An atheist doesn't believe in God by definition. Granted, there might be a few who call themselves atheists who actually believe in God, but keep in mind that the vast majority of atheists aren't activists trying to persuade people to give up their religion.
Heck, I'm an atheist and I celebrate Christmas. Speaking of which...
Christmas, if you really get down to the brass tacks of it, isn’t about reindeer, elves, iPhones or Lindsay Lohan punching a gypsy, but about mankind’s sin problem and what God did to remedy it by sending His Son.
Increasingly, that is what Christmas is about (er, aside from Lohan's gypsy slur). Christmas is a far more commercial holiday then it was few generations ago. That's not to say that millions don't still celebrate it as a religious holiday, of course, but for millions more (myself included) it really is all about Santa, lights on trees, a special dinner with the family, and the giving and receiving of presents.
I too am a non believer(not sure believer????) of Christian(RC) descent. I do not call myself an atheist as I do not believe I can know with any more certainty than those of faith can know that, as a demonstrable fact.
I too celebrate Christmas. those who do not believe I should do that....can sue me.
Why are some atheists so serious about their beliefs? They're as bad as some devoutly religious people I know. Lighten up, everybody. It's Christmas!