Part of the reason is that it's fun to read someone smart confirming your own opinion. Humans are tribal creatures, for almost no reason we are fanatically loyal soccer teams, political parties and Roger Federer. Once we've picked our side, we distort reality in its favour and we always like to see our side win. This isn't admirable, but it's pretty difficult to shake the instinct if you've been sucked in. Atheism is just as prone to this kind of loyalty as soccer or whatever.
All this is pretty negative on buying these books but I don't want to diminish the entertainment value they provide even if it isn't noble. I also don't think it's a coincidence that there are suddenly lots of atheism books at a time when religion has suddenly become much more prominent in public. The more I bang on about something position I feel strongly about the more likely it is that'll I'll hear from people who disagree. It makes sense.
Now does anybody deny that the world seemed a more religious place after Bush came to power and September 11th happened?
This post was inspired by Steven Levitt, who is not concerned he might upset people by telling them that abortion reduces crime, but is concerned that people might think he's anti-religious.