This is the first move in response to what Apple is doing with the AppleTV. AppleTV currently has a small foothold in gaming... but it's growing.
Apple has shown, with iPhones/iPads, that it can put out a revision of their hardware every year and keep backwards compatibility with a huge App library. After a few years/revisions the AppleTV will be just as powerful as current-gen consoles and (just as we saw with mobile gaming overtaking mobile consoles) AppleTV will begin to suck dollars out of the established consoles.
Apple can (if they want to) iterate faster and provide a better ecosystem than Microsoft or Sony... and Microsoft knows it. Microsoft wants to get ahead of the game by switching up their model to allow for faster hardware iteration.
Will be interesting to see where this all goes...