But Apple compels you to use their payment platform even if you already have access to another one.
Take the Amazon app. You can happily check out a basket for tens of thousands of dollars using Amazon's checkout screen and there's no issue. But try to add an ebook to your basket. You can't. Because Apple has decreed that ebooks are digital goods, so they have to be subject to the Apple Tax.
They've acknowledged that they have no problem with you using Amazon's payment screen, so why must they have a problem with using that same screen for a different item from the same store? It's because if they were to allow Amazon to sell ebooks through their own payment portal, they'd have to allow all digital goods to be sold via external payment gateways, and they'd lose that sweet 30% of all those transactions.
So they've had to create this artificial distinction between physical and virtual goods, and keep having to come up with creative ways to continue justifying that digital goods are somehow different and they absolutely have to be processed via Apple's payment mechanisms or the world would fall apart.
I have no problem with Apple providing a payment gateway, and for charging a 30% fee for using that gateway, even though 30% is daylight robbery. I do have a problem with them forcing you to use that gateway and no other.
Besides, Apple's already been in the news in the past few weeks for trying to compel Wordpress to add purchases to their free app despite the fact that it didn't sell anything. So in this case, Apple was trying to compel Wordpress to pay them a royalty for their apps, despite Wordpress not receiving any payments from the app. Regardless of the later retraction and almost unheard of apology, it still is a case of Apple compelling a developer to pay them a royalty despite not being in any way entitled to one.