The reasons for these decisions and others have been discussed in an interview with Barry Collins of Microsoft, who answered many questions in relation to this new service. You can easily read all of this as a step backwards. Not only is Microsoft admitting that you will be unable to transfer the music from your mobile device to a PC, but they also admit that you won't be able to transfer them to a new mobile device.
Given the average lifespan of a cell phone is going to be a lot less than what you'd hope your music collection to have, what's the incentive to purchasing music through this service? Buy a new phone or lose your old and say goodbye to your purchases, which came at a premium to begin with. They can change all of this by stripping the DRM from the music and making transfer between devices possible, but they sure don't seem interested in doing that.