There is, but the way the Apple app store rules work is that if you are purchasing something for use in the current app, it must go through the IAP API and give Apple their cut. Purchases for things outside the app (like physical items) are fine.
The way this works with Amazon, is in the Amazon store app a user can buy ebooks/audiobooks (along with anything else Amazon sells), and then those books are sent to the Kindle app on iOS. There is no option to buy content directly in the Kindle app.
I also believe the apps cannot directly link to each other as that would circumvent the above.
EDIT
I just checked (it had been awhile) and it looks like you can no longer buy the books within the Amazon store app either, and can only get a sample.
If anyone at Apple is reading this, this creates ridiculously bad UX for iOS users.
Search for a book on Google. Click on an Amazon Kindle Search result. The Amazon iOS App launches and tries to find the book, realizes it's a Kindle result, launches Safari to show the result.
So the was trasitioned from Safari -> Amazon -> Safari pretty much because of petty politics from Apple. And all for what? A happy Kindle user isn't going to suddenly switch over to iBooks and you've just made life much worse for him.
The pettiness in the mobile platform wars is unbelievable.
So basically all that Valve has to do is to redirect everyone that wants to make a purchase through Safari to the Steam web based store. Then it would be doing all of the selling on the web like a normal store, but content consumption could be done anywhere.
You're absolutely correct. It's been awhile since I have bought anything not on Amazon's site. At one point I thought I could buy books in their app, but it's possible that was only samples. And yes, it's comical how bad the user experience is.
Ok. What if Amazon customer buys a kindle ebook on Amazon.com and then reads it on iOS Kindle app? I don't imagine Apple gets a cut in that scenario, do they?
A comment here said that Valve was open to disabling buying in the app and they are still banned. Hmm.
I can see how Apple's position to get some cut is reasonable, but we are talking multiplatform here. Was Valve unwilling to give Apple any cut at all? Was Apple insisting on too big a cut? What's going on?
As I edited above, it looks like Amazon changed this (or maybe it was always like that since I always use the website). According the Apple documentation though, a user should be allowed to bypass IAP for purchases that will be consumed outside the application. See:
The way this works with Amazon, is in the Amazon store app a user can buy ebooks/audiobooks (along with anything else Amazon sells), and then those books are sent to the Kindle app on iOS. There is no option to buy content directly in the Kindle app.
I also believe the apps cannot directly link to each other as that would circumvent the above.
EDIT
I just checked (it had been awhile) and it looks like you can no longer buy the books within the Amazon store app either, and can only get a sample.