Besides the fact that they'd lose some money from the publishers, I will never understand any single argument from Apple about Spotify/Netflix and the rest being prevented from adding their own billing services, even if it means just letting the devs add a link to take them to a web page.

This, especially when Apple is trying to compete in the same field and has an unfair advantage on their own turf.

And yes, every other Store should do the same although from my personal experience and from that of the company I work for, we've only had issues dealing with Apple so far.