But the "receiving" ISP is only receiving the data because its users asked for it. It doesn't make a difference if the data comes from a Spotify server or from a PC. It's still the same amount of data being transferred.
Receiving data over a transit connection may be more expensive than from a CDN. In DOCSIS networks, P2P also tends to introduce last-mile upstream congestion which is expensive to fix.
The last mile problem is presumably exasperated by P2P users being mostly consumers, rather than businesses, and therefore paying next to nothing for the connections.
