Disbelief in something doesn't require faith.
For example, I don't believe that Vulcans (Spock's people) actually exist outside of fiction. That doesn't mean I have proof that they don't exist. It means that I don't base my life-choices around the idea of Vulcans existing.
Now, change Vulcans to YHWH (or any god), and you'll see what I mean.
EDIT: People who have faith in something, whether it's another person's word, a god, or what have you, tend to base their life-choices - or at least some of them - around the thing they have faith in. Having faith changes their lives in some way, whether for better or for worse. But atheism doesn't do that. In order to be an atheist, you either have to not believe in gods to begin with - in which case talking about it changing your life is silly - or you believed in some god and then stopped doing so. In that case, it's the loss of belief that causes the change.
But how can a loss of belief be called faith? It's not just an oxymoron, it's actually the antithesis of the term.