No small surprise they want to block it off. As another said, you can still use Steam on Windows 8. You MUST use the app store to get stuff on their tablets, which is why I won't be using one of their tablets. From my perspective, they are far far far behind the curve on this. My buddy bought a Windows phone, and while it's a nice piece of technology, there is nothing on it because no one will develop for MS's mobile platform. All the fans say it was just a test bed for an app store, but they lost customers by using them as guinea pigs. My buddy won't buy one anytime soon.
While I'm ranting, I'm just about done with tablet or phone games, they are mostly complete shit, a lot of them aren't even worth the dollar they cost. No one really makes legitimate gamer games for them, and while there are examples of them getting it graphically done, the games themselves are horrible. I found a fun FPS style game that is impossible on touch screen controls. I found another that was pretty sweet, until I had to spend real money to progress in the game. And ultimately, that game was shallow as fuck and had very few tricks up it's sleeve. I think about all my favorite games and they can never be represented on a phone. And yes, while it may take time to get something like controls right, in regards to what any other gaming platform does, they are moving at the slowest pace in respect to progressing positively.
/end rant
Back OT: No biggie, they will support Windows 7 for a few iterations more of Windows. And by the time they really start to restrict the system, Linux will be ready for gamers.