I agree that if the end points are inside the ISP, it's good.
But peering is different from transit; peering is a delicate relationship where ISPs route traffic to each other for "free", as long as the demands are not too asymmetric. If this software results in a lot of traffic generated from a peering ISP, then it could be causes of worry for the receiving ISP.
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.
But peering is different from transit; peering is a delicate relationship where ISPs route traffic to each other for "free", as long as the demands are not too asymmetric. If this software results in a lot of traffic generated from a peering ISP, then it could be causes of worry for the receiving ISP.