It matters who made the App Store and facilitated the app in question. If more iPhone users are downloading the Kindle app than Android users by a wide margin,

It matters who made the App Store and facilitated the app in question. If more iPhone users are downloading the Kindle app than Android users by a wide margin, then that means that Apple offers a higher value to Amazon. You're oversimplifying the relationship but the fact is that Amazon's contract with Apple was based on Amazon's assertion that it would only be existing Kindle users that would want to download the Kindle app and that provided value to Apple. When that was no longer the case, the agreement ceased.