They've positioned themselves so that none of their IP even matters as far as their core business goes. What does matter is that they own the marketplace for games. Giving the source code away only allows people to improve the platform, which only strengthens their hold on the market place.
I'm not sure if they're open sourcing the actual code for steam... it sounded like just the OS. None the less, code doesn't make market places. It takes effort, good faith, proper marketing etc.
