I don't use Spotify anymore, but when I did I was completely aware that this was happening, and ultimately I'm not too bothered by it using my connection to support other clients, because that's partly how it starts playing so fast. I would be very upset if it was using my entire connection.
Spotify should cap the speed at like 128KB/s, because that should be plenty for what they are doing. If they were using my full connection, I would be furious, because I'm not a data center for them, and I know how much data centers charge for bandwidth, and they would be getting an insane deal (my home connection is 100/100Mb).
It should probably have a cap for how long they are using your connection also. Once you've shared as much as you've downloaded, your connection should either stop sharing or go to an even lower speed.