Windows 8 isn't had for gaming, it's just bad for Valve.
Windows 8's store is quite probably a violation of antitrust laws, but they're managing to break into grey area by locking it down only for "tablet" style apps. Here's how it works, Microsoft is doing exactly what Valve is doing but tying it to their existing desktop OS monopoly such that regardless of if Valve produces a better product for end users, Valve will still lose in the market. They way Microsoft is doing this is by making it impossible for end users to load "Metro" apps that use the tablet interface unless they are purchased through MS's built in store. This means while Steam can still sell apps that use the existing Windows UI, they won't be able to sell apps that work with the other half of of the UI APIs including touch screen capable apps and apps that target both touch screen and keyboard. This provides MS a huge advantage (and of course the Windows store is pre-installed just like IE) without being a direct violation of existing antitrust rulings.
The fun thing is, they can drag this out for years in the US courts because they previously defined the market in terms of desktop OS's and what MS is trying to do is simultaneously merge the desktop and tablet OS markets, while taking an action that is only legal if the markets are already the same. The EU will probably slap them down and may or may not take any sort of effective action before another market loses all competition.
Valve's just concerned with their potential market being at risk.
True, but free competition in the market is a huge concern to us consumers as well. I like the innovation, lower prices, and better quality that comes from having multiple vendors competing for my business instead of being stuck with the one and only vendor that can treat me like crap and make me pay through the nose for the privilege.