Quite simply, theism is the majority (particularly in the U.S.) An atheist could walk through any town and, without even really trying, stumble upon a church. In this way, an atheist can't really 'forget' about god, when there are constant reminders everywhere (also on money and the pledge of allegiance).
It's true that an atheist doesn't have to talk or debate about god, but considering they are the odd ones out, there is an underlying need to defend one's chosen viewpoint, not because they feel it is flimsy, but because, if it's brought up, few understand their choice. If I announced to a room that I'm an atheist, there is a good chance that most people will ask me why, or where do I get my morals, while a small number might even hate me, thinking that I'm 'what's wrong with America'.
Granted, the latter group is not the majority, but you can see how an atheist might want to talk about it so as to spread awareness. We aren't abnormal, we're just different. But, as the minority, it falls on us to spread that awareness because the majority has no incentive to learn more about us.