Here's the thing with Spotify: The major four all own a share of the company (around 4-6%, if I'm not mistaken). For them to be willing to license their music, they negotiated a sweet deal: They get a lot more per play than indie artists (or artists on labels not on the major four).

So, if you're a Spotify customer, paying 100 SEK ($15)/month and listening exclusively to, say, Santigold, most of the loot will still end up in the big four's pockets.