Spotify wants to use Apple's distribution channel without paying for it.
That's unfair and doesn't reflect what happens in the real world. If I sell my software through a retail store they still take a cut.
Apple has a store. Amazon has a store. Who is to say that Amazon's portion of operational costs and profit isn't the premium?
Amazon could choose to have a Kindle experience which doesn't go through the App store, such as a HTML 5 app. They distribute in the store because of the discoverability (and likely because they prefer to have a native app that can enforce their store's content DRM.)
Amazon could choose to expose more functionality through their app, such as title search, being able to look at reviews, download sample chapters, and even be able to fetch new books through the kindle unlimited and other prime features. They prefer to drive everyone to their own store instead.
You can do title search, look at details of books, see reviews, download sample chapters, and download Kindle Unlimited books directly to your library with one click.
I presume what you _can_ and _can't_ do is directly dictated by Apple's policies, not what the Kindle team has been able to implement. As others have pointed out, native apps are a far superior user experience to HTML "apps".
Sadly, due to Apple forcing everything to use Safari's browser engine, PWAs are redheaded stepchildren on iOS: https://mobile.twitter.com/jbogard/status/104788904642763161...
