Well, Apple just want to "grow the pie."

Although growing on the pie seems a more apt metaphor. Defending 15% fees on second-year subscription is pretty difficult. There's no marketing, very little fraud risk. It's really rent-seeking behaviour – which of course is the commercial point of the walled garden approach.

This is Apple exploiting a very lucrative market position, one that they did invest quite a lot of money in – and took quite a gamble – to create.