Valve are shitting bricks about the Windows 8 App Store because it's a direct competitor to their app store, Steam. I doubt they hold the Mac App Store in particularly warm regard, either.
This has nothing to do with Free or Open or anything like that; it's about ensuring that Valve's business model is protected, by having people play games without the temptation of the Windows (or Mac) App Stores, and rather having people send their pounds sterling (other units of exchange are available) to Valve, instead.
The problem for Valve is that there are very few games that run on Linux, and when it comes to AAA titles (think Call of Duty, etc; the titles that sell by the million) that number drops to pretty much zero. The workaround seems to be that you stream your games-that-don't-run-on-Linux (i.e. most if not all of them) from your actual Windows PC to your SteamOS box.
At which point I would suggest the vast majority of people are going to go "eh, fuckit. I'll buy an HTPC or an Xbox or a PlayStation."
The comment sections of websites reporting this are alight with starry-eyed gamers who are convinced that this heralds the coming of Half-Life 3, but the harsh reality is that - unless the best industry cover-up of all time is in effect - this game is not in development. There are no current AAA titles for Valve to bring to SteamOS.