Why should they? They don't have any reason to allow mature games, which certainly haven't been very common on consoles. It looks like PCs are headed in the same direction. Chances are these sorts of products will simply follow the path of least resistance and move to the web.
Edit: my bad, I'm not a big gamer, so I'm not familiar with the ratings terminology. When I said "mature" games, I was referring to games with adult content, not the games where you can shoot/kill people.