Amazon doesn't block customers from buying books from elsewhere and reading them on Amazon devices. It's fiddlier than buying through the integrated Amazon store, but it's a fully supported way of using your Amazon device.

I'm not aware of them blocking sellers from listing books on Amazon and also at a lower price elsewhere (though it wouldn't surprise me if they did), and they certainly don't block sellers from telling their customers how much Amazon's cut is.

You do run into a bit of a problem though: Amazon wants their cut, Apple wants their cut, the credit card company wants their cut, the publisher wants their cut, and there are likely others in the chain as well. That's not to say that each party doesn't deserve payment for their services, but there does have to be some limitations on how much they can take.

And that limit should reflect how involved they are in the existence of the product. Is a credit card company intrinsically vital to the existence of a book? No. Then their cap should be reasonably low, like in Europe.

Is Apple intrinsically vital to the existence of an ebook?