I assume it's to pass the plate.
this is very weird. I've always been suspicious of "organized atheists." I know dozens of atheists (I'm in a PhD program) and while there are a few who constantly bash believers and seem more obsessed with religion than most religious people are, most of these atheists keep their unbelief relatively private. They don't let it define them, they don't make a big deal out of it, and they don't really care if other people disagree with them. In general they are very individualistic people and their atheism is an outgrowth of that. Joining a group devoted to promoting atheism would be the last thing most of these folks would want to do.