If you buy music in WMA format, you are locked in to using WMA-format, whether or not it has DRM doesn't matter. It would be easier to re-encode non-DRM'd WMA in to another format, but does it really matter?
Your average Joe won't bother to re-encode his WMA-music in to another format, even if there wasn't DRM. Someone who cares about sound-quality won't do it because re-encoding in to another format will cause sound artifacts. An advanced user who wants to re-encode the file isn't going to be stopped by DRM, because circumventing it is trivial if you are willing to re-encode (the hardest thing is stripping a file of DRM without re-encoding, and in that case the WMA-file will still be WMA, just without DRM).
I must admit that it is true that if a big recording company allows everyone to sell music in non-drm'd format, 3rd party music stores (napster and the rest) won't have to use DRM'd WMA (which is pretty much the only choice for everyone except Apple), though. They could just sell the files in MP3. But Microsoft can't change a recording company's mind, so Microsoft has to jump aboard as quck as possible. They would have certainly preferred to be first (and perhaps they could have tried to make a deal to make it exclusive, at least for some time), but second is okay, too.
They will do everything to give WMA the edge over other formats, or at least keep it in an equal playfield with the others. Offering non-drm'd music in WMA is doing exactly that. It isn't about mimicing Apple.