Amazon's spokesperson Cat Griffin in an interview with Ars Technica:

We 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.

Sounds logical, but Sony Music begs to differ.

Avoiding a license negotiation with large record labels might have given Amazon the advantage to release its cloud service on schedule and sooner than its competitor Google. Google announced a similar service at its developer conference “Google I/O” almost a year ago.