From what you're saying it sounds like Spotify should have built a smartphone platform. It's not free for Apple to build and maintain the App Store and everything that entails—from API documentation to the rock solid software platform that is being kept up to date and is astonishingly robust compared to any internet-connected consumer platform that came before it.

Some apps pay for that effort with a cut of the app's retail price. Spotify pays for it with the tiny fraction of their users which (a) pay for it at all and (b) pay for it through their iOS device.