Why the hell not, eh? Do you want atheists to stand outside the group, disapproving of all that celebration and cheer, because of some idea you have about what an atheist is? Oh, wait a minute, it's JWs who stand outside the group disapproving.
Also, Terry, some of us atheists actually have fun at christmas -- no need to fake it. One of the advantages of being an unbeliever is that I can choose which bits of holidays I like, and ignore the rest. So I do the fun bits, like baking goodies and giving little surprise presents to the neighbors, that sort of thing, while ignoring the commercialism and the religious aspects of christmas.