having an app available on a very popular OS is leveraging the success of that OS. in this instance iPhone.Providing the access is different than deserving credit for someone else's success. You have claimed that the iPhone is responsible for Spotify's success, that is a different claim than saying that the iPhone provided access to the market.
the iPhone is Apple's success. And Spotify are very happy to let users download their app from there. they just dont want to pay anything for the privilege.
And Apple happily let them put their app there for free so long as their is no In App Purchase going on.
That's a pretty fare deal. How many Apple devices is Spotify installed on? Someone must have stats.