They have every reason to allow mature rated games. Mature rated games are among the most popular games out there on console and PC, Grand Theft Auto, Call of Duty , Elder Scrolls to name but a few.
PC Gamers who like R rated games have been a fair chunk of MSs user base for years now, thinking all the way back to 1991 and Wolfenstein 3D. Games are a big reason a lot of people are sticking with Windows as opposed to getting a Mac or trying Ubuntu.
Another factor to consider here might be that not all R rated content is considered equal, for example it appears that in America strong violence is less taboo than sex or swearing. So maybe Blood'n'Guts 3 gets an exception but Karma Sutra: The Game doesn't?
I also think it'll be quite some time before we have AAA game engines running nicely under WebGL.