I don't think anybody is arguing that you should have to pay continuously for ownership of a song you bought. But if we extend the Macbook metaphor to Spotify's case: a Spotify user didn't buy a Macbook, they're paying a subscription fee for the freedom to use a wide variety of computers, and the subscription service compensates the computer manufacturers based on how many users choose to use their model. This incentivizes creators to create good things, so that users pick their creation, so that they have income.