I don't think Microsoft is doing it for the Open-Source spirit, as many seem to believe. They have a long history of attacking/smearing open source and displaying a "I heart Linux" slide is not sufficient to erase all the FUD and their questionable software patent licensing schemes (which are completely anti-open). They called Linux a cancer, even.

I think they're approaching this from a redundancy point of view, if person X already has Windows on his desktop (on which they have the market cornered), provide them with subsidized Windows platforms on other hardware where the monopoly is not yet in place. This then would cause the interest in other platforms to dwindle, as people would flock to what they know. Here also the EET mantra could come into play again, by offering features not available to other platforms.

They need a foothold in other hardware, most likely for the coming of Internet Of Things.

It makes business sense for them to increase their reach, but don't think that equals Microsoft being fond of Open-Source.