When they opened their high-quality audio service the songs had clear artifacts of having been encoded more than once. If you had access to the original uncompressed audio stream, you'd simply encode a higher quality version. Many albums weren't available for high quality streaming at all.

The logical explanation was they didn't have a database of lossless audio from which to re-encode, and we already knew they were using P2P. Why not use the P2P to raid users music collections as well.