...if atheist's really didn't care they'd have no beliefs at all. that they believe something in the first place...
It's interesting that you should bring this up in this way. Personally, I feel that the definition of "Atheist" is as follows: A person who holds no belief in a deity.
Not believing in something does not mean I believe in something else. For example, I don't believe in Santa Clause. Would you say I have an opposing belief? I wouldn't. I would just say "I don't believe in Santa Claus." Not believing, in my estimation, requires no action on my part. Believing, on the other hand, does.
Generalizing is always tricky, though. Isn't it? Not all theists evangelize any more than all atheists do. So, of course, I only speak for myself and the majority of those I have spoken with on the topic.
Atheist don't hold a belief. That is what makes them Atheist. Finally, not holding a belief is not synonymous with not caring.