If Apple lets Amazon do this (presumably because Amazon is too big to ignore) and won't let Steam do it (because Steam isn't big enough), then this feels more like a cartel than monopoly.
The way this works with Amazon, is in the Amazon store app a user can buy ebooks/audiobooks (along with anything else Amazon sells), and then those books are sent to the Kindle app on iOS. There is no option to buy content directly in the Kindle app.
I also believe the apps cannot directly link to each other as that would circumvent the above.
EDIT
I just checked (it had been awhile) and it looks like you can no longer buy the books within the Amazon store app either, and can only get a sample.
A comment here said that Valve was open to disabling buying in the app and they are still banned. Hmm.
I can see how Apple's position to get some cut is reasonable, but we are talking multiplatform here. Was Valve unwilling to give Apple any cut at all? Was Apple insisting on too big a cut? What's going on?
In my other comment on this thread I've pointed out how this creates hilariously bad UX for iOS users.
https://developer.apple.com/app-store/review/guidelines/#in-...
https://developer.apple.com/app-store/review/guidelines/#phy...
Admittedly it has been a few years since I was deeply in iOS development dealing with this.
Search for a book on Google. Click on an Amazon Kindle Search result. The Amazon iOS App launches and tries to find the book, realizes it's a Kindle result, launches Safari to show the result.
So the was trasitioned from Safari -> Amazon -> Safari pretty much because of petty politics from Apple. And all for what? A happy Kindle user isn't going to suddenly switch over to iBooks and you've just made life much worse for him.
The pettiness in the mobile platform wars is unbelievable.