I'm not religious so spare me that accusation, but if Christmas is originally a Pagan holiday, and that's your excuse to celebrate it, you're definitely Pagan, right? No, really. You say you celebrate Christmas because it's not a Christian holiday, but a Pagan one. Now tell me, are you a Pagan? If this thread title was really what bothered you, ask yourself how you'd answer to "Why do Atheists celebrate Pagan holidays?" I'm pretty sure I celebrated Christmas less than most on this forum, so quit your bitching before you start it.
Obviously Christians aren't celebrating it as a pagan holiday, and atheists aren't "celebrating" it as a Christian holiday. It seems like people, like some who posted in this thread, do because it was a tradition they grew up with. So a lot of people view it just as a secular celebration, not related to Christ at all. It's popular in Japan, but it's all about gift exchanging and Santa and all that. .