Cat Griffin, Amazon spokesperson, via ars technica:
Cloud Player is an application that lets customers manage and play their own music. It’s like any number of existing media management applications. We do not need a license to make Cloud Player available.
[W]e do not need a license to store music in Cloud Drive. The functionality of saving MP3s to Cloud Drive is the same as if a customer were to save their music to an external hard drive or even iTunes.
If I purchase music, store it in the cloud, and listen to it on my computer later on, what’s the problem? I love this cloud MP3 player.