I thought surely you can buy ebooks on the Kindle app for iOS. But a quick search and you are right - nope, you cannot buy books on the Kindle app for iOS. The option just isn't there.
And you can't even present a link to purchase digital goods through a browser... You can open Mobile Safari and search Amazon for a Kindle book, but you can't use an Amazon app of any sort to do part of it.
I just downloaded Bandcamp for iOS and under Digital Album I see "Not available for purchase on this device" along with links to purchase the physical vinyl or CD.
I'm not sure. I've found this in the App Store Review Guidelines:
>3.1.5 (a) Physical Goods and Services Outside of the App: If your app enables people to purchase goods or services that will be consumed outside of the app, you must use purchase methods other than in-app purchase to collect those payments, such as Apple Pay or traditional credit card entry.
But if this applies to Bandcamp, then it would equally apply to Amazon... So I don't know.