You are paying for music licensing and you get the delivery for free because of the p2p nature of their system. Traditional client-server delivery would've simply translated into higher monthly fees, which I am pretty sure would've not been any more popular among their average user base.
Your upload bandwidth is more limited than down though (although how much does the average user really use upstream anyway?).
It should be easy to use a firewall to throttle the outbound traffic on whatever port spotify uses. Pherhaps you could just block it altogether , does the service still work even if you are not sharing?
I wonder what the legal implications of this are though, could the RIAA or similar at some point decide to start suing individual spotify users for sharing their music even though the record companies have some agreement with spotify in place?