> 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.

If that was the case they could have just passed some handling fees to the indie artists -that would still be much more reasonable than the fees that the artists would have to pay to a publisher. I think the record companies -still in the process of understanding and adjusting to the new business model that Spotify established- pushed for this, one way or the other, to ensure that Spotify doesn't become a competitor like Amazon became to book publishers.

Distributing your music isn't really financially burdensome as is. Most distributors cost in the range of $20-$30 per year. Some are even free.

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

It all boils down to that Joe Blow is an unstrusted actor and big label is not. They needs to check that Joe Blow is not uploading Bieber's latest song as his own and that all the metadata is correct while they not need do that for most labels.

Your assumption that they go through the same work flow might be mistaken.

The same type of questioning applied to banks and transferring money:

Why would it cost a bank more money to print and mail checks to individual people when they already do balance transfer payments every night to other banks and branches?