Because it can be turned off:
Microsoft last week told former MSN Music customers that it would no longer support the DRM licenses for tracks bought through the failed music sales portal. The news meant customers would lose the ability to play the music they own once their existing computers ceased to function.
iTunes Plus, Play, Amazon (when it gets to the UK), fine. But with DRMed content, even from a popular service like iTunes, you never know what’s around the corner. The default state of the file is unreadable without their permission.