Who's willing to bet that the new "Bing" edition is for exactly this purpose? A lower price point supported by more heavily pushing the online services.
Citing that MS is moving in other, not Windows related services, also to push for online use, to an extensive cost(!), that could simply be the fact!
They're trying to muscle their way onto the app store market. Having more people using your platform means more app store revenue. Even if you have to basically give the thing away to get people to use your stuff, they're betting it will make them more money in the long run. I wouldn't exactly be opposed to the idea if they actually had good games with controller support for a decent price rather than the stupid touch-screen crap that noone wants to use.
From what i read 15$ per license is only for OEM:SLP core editions on devices costing up to 250$, so no BONER here.