The industry as a whole is moving into walled gardens right now, following Apple's lead. I'm very worried about that myself, so I'm cheering Valve on here.
It's true, having open platforms aligns with Valve's business goals. I fail to see the problem with that, it's a lucky coincidence.
Valve wants to build it on top of am open platform (Linux) because building a walled garden inside someone else's (MS's) walled garden is risky. But that doesn't make steam itself an open platform.
Steam is a more like a walled national park - the walls are there, but they give you a lot more room to run around.
At some point, open platforms might no longer align with their business goals. That's when they'll do nasty stuff, and that's when I'll whine about them. Currently, I'm cheering them on.