I think hackers steal other people's credit cards info and buy loads of keys with them to sell on. People find out and get a refund from their bank so the devs/ pubs receive no money, but the keys are already sold and people have the game. I'm not sure if stuff like this has been cracked down on yet. Anyway, for reasons other people have said and more these sites are just bad.Found this.
Is CD Keys Legit? Thoughts & experience with them? : Steam