What makes you think that 3% cut is enough?
But Apple can ask whatever they want. They can't block side-loading though. That's the uncompetitive part.
You can run your store and pick whatever terms you like. You can't use your marketshare in hardware sales to bundle a forced store.
Imagine Tesla charging you 30% of any grocery shopping (i.e. would refuse to open the doors if the store didn't share 30% of its gross revenue).
I mean, its literally, textbook anti-competitive. The App Store as a store isn't competing fairly, on its own merits.
Also keep in mind, that this whole getting raped with transaction fees is a 'america-only' thing. This is much better regulated in the rest of the world.
Specifically the costs are fixed, so anything that is a percentage is just fucking nonsense. It doesn't cost more to charge 5 euro's than it does to charge 1 euro. It uses the same electricity, the same personel costs. There is a point where its get more expensive because of risk management, but thats above 100 euro per transaction.
Percentages on transactions are generally only allowed when its a loan. Which is why Americans are always buying things with credit cards ("loaning the money"). Most people pay for things with their own money, not with a loan. (i.e. direct bank transfer). And those transactions have a fixed transaction costs. Worst case 1 euro (low-volume, your personal webshop) all the way down to 5 euro cent (high-volume, i.e. the supermarket).
So explain to me where the hell you get your 3% from? You just sound like an already boiled frog saying 'are you sure we can survive in cold water?'
They're not just a payment provider though. They offer infrastructure, promotion, a huge locked-in userbase with the means to pay for software, etc