No idea why MS is doing this but...will happily take.
It's probably like when magazines sometimes give away free subscriptions when their readership gets low. In order to prove that Xbox music is viable for record labels, MS has to show usage. Certainly MS is picking up the bill for all the downloads.
IMHO, it is the best music service available and I've tried almost all of them from the internet radios (iHeart, Pandora, LastFM, Nokia MusicMix) to the ala cartes (Beats, iTunes) and I keep coming back to Xbox music. It isn't perfect (which is why I keep searching) but it offers the best radio experience with smartDJ; the best streaming experience with Xbox music subscription; and a lot of really great offers for buying DRM free music. It still suffers with selection though. It has about 80% of the music I want to listen to.