Sony can get around this issue quite easily by just launching their PC titles on a new PlayStation PC Launcher similar to the Xbox, EA, and Blizzard launchers that won't be available on the Xbox hardware.
They'd have to pull their current titles to prevent what's already on it from playing though, which is also a possibility.
This is Microsoft forcing Sony to play ball. They can either just sit and let this happen, or prevent it, but lose possible market share because an Xbox/Steam machine is undeniably enticing.
This move ultimately though may end up destroying the console market in the long run. Xbox consoles as we know it are already being phased out and will be looking more like a walled off pre-built PC marketed as a console solution. I'm sure TV compatibility will be part of the marketing even though custom PC's can already do that.