You're paying for other features... let's say the cost of Spotify not being P2P increases the price by N dollars per month. The "free" users pay N, paying customers pay $5+N or $10+N. I think it's different to complain that they don't offer a "non-discounted" payment plan to opt out of P2P.

(I suspect N is rather large)

Not exactly. If you have the premium ($9.99/month) version you can enable 'offline' mode on playlists which will download them to your phone/computer/tablet and not use your network.