Huh, interesting... You know, I've never really wondered about Humble Bundle specifically, but you're right, they seem to be selling your run-of-the-mill Steam keys, or at least you can activate them effortlessly in Steam. Maybe it's a case of Steam themselves handing out keys (instead of the publishers) to increase user retention? I honestly don't know, this is all just speculation.

I actually didn't click on your link at first, because I assumed it would just show other stores where you could purchase the whole game instead of a key, so I'm sorry that you had to clarify that.