From MacRumors: “All Things D points out that having official licenses can allow Apple to store a single master copy of a song rather than storing individual copies for every user. Amazon’s original argument against needing the licenses was that their service was the same as any upload storage service. This meant that users needed to upload copies of their old music to be able to stream them. With the proper deals, Apple could avoid the need to upload individual copies and simply allow users to stream off of the single master copy. This could save on significant upload time for the user and storage requirements for Apple.”
Not to totally plug it, but I’ve really loved Amazon’s cloud music storage/playback service. It has it’s problems, which are UI-related, but otherwise it’s been great to not have iTunes gumming up my computer while I work.