It has always seemed weird that if I use my Spotify account on my Android phone, iPad, and desktop, who gets a cut of my money depends on which one I use to set up billing.
For services which exist and function perfectly well outside of the Google/Apple binary, why should they get any more than a nominal fee for hosting the app download?
When apple gets control over the transaction, they can take whatever cut they want
They could have a standing charge for each app, with charges per thousand downloads, for instance, which would more closely follow their actual costs, and charge 5% for payments. That would have a different set of positives and negatives, but the way they've chosen is just one way amongst many options they've got.