I guess that would be the logical thing to do. There's probably some synergy at work. If these games could be widely promoted, maybe their average value to Valve would be $10k each or something. Instead, they probably net 1/10th of that before they drop off the radar completely. Building in a sub-system that guarantees that certain games can only be bought with certain methods of payment seems like a pain in the ass. However, they could do it. And that sort of argues against the idea that you'd be building yourself any kind of moat by setting up a game platform for just the XXX stuff.