This is wrong.
If it has no reliance on the infrastructure, why did Amazon make an iOS app? If you're correct and there's 'no reliance,' then they'd just have a web-app, right?
So clearly you're missing something.
What you're missing is that being in the app store brings millions of users and viewers and eyeballs. It's free advertising on a massive scale. How many millions of people downloaded the Kindle app who never would have thought to type in
www.amazon.com to check it out? Each and every one of those viewers is there
because of Apple's software.
That's called Amazon making use of Apple's infrastructure and it's pretty huge. It's exactly why everyone's rushing to get their own app in Apple's store.
To ignore that fact is to close your eyes. It's pretty obvious that that's why all these companies are making apps. It's not just for fun.