> However, that same system prevent me from buying Kindle books from the Kindle app on device, for example. Even though I can open the browser on that same device and buy them from Amazon.
I’m sure Apple would actually prefer that you buy the Kindle books on the app, even if they didn’t get a cut of the sale. It is actually Amazon choosing not to do it in order to dodge paying the payment processing fees.
Not a single payment processor in the world has a 30% fee while also having a competing product in which they can have the prices arbitrarily low (because they don't care about the 30% fee they pay to themselves).
The egregiously high processing fees from AmEx are at 3.30%.
I’m sure Apple would actually prefer that you buy the Kindle books on the app, even if they didn’t get a cut of the sale. It is actually Amazon choosing not to do it in order to dodge paying the payment processing fees.