> Why does it cost more to Spotify to let the unknown-Joe-Blow-not-signed-to-a-label upload his 4-minute song?

Unless I'm mistaken, Spotify would get the new Bieber automagically via an ERN (Electronic Release Notification) delivery which contains asset links, artist, meta, rights information etc. Since that's from a trusted* source, it can be ingested as-is and everyone is happy. For independent uploads, there's almost certainly several steps of human review and possibly interaction - which costs money and time.