Didn't Batman throw Joker off the building in the first place? If he let Joker fall then, Batman actually would have killed him. That makes it sorta-kinda-maybe-not-really distinguishable from Batman Begins, where apparently blowing up the train tracks and letting the person in the train crash and die doesn't involve actually killing him. I guess it wasn't so direct, at least.
To add to my last post, I think that's just what a lot of people think superheroes are supposed to do: save everyone. I disagree with such an oversimplified philosophy as well, but comics are well known for their black-and-white portrayal of issues.