I never said anything of the sort.
I simply said Apple does not do NOTHING. When you buy something through the Amazon app, Apple's servers ARE contacted for the request and the response of buying the 'in-app purchase'. Apple DOES do the credit card processing and eats the credit card fees. Apple DOES calculate sales tax and spends money to file with the proper states. Apple DOES transfer the 70% money to Amazon's bank accounts, which includes a bank transfer fee that Apple eats. Apple DOES discount their iTunes gift cards, which means the full 30% doesn't always go to Apple.
Apple takes the 30% to pay for these activities, plus as a 'convenience fee', - that you as the consumer can easily avoid Apple taking by going directly to the Amazon site on Safari and doing the same thing. Apple will get 0% that way.
Apple does that because Apple FORCES using its payment system so yes, why don't take a cut of ebay, Amazon app. It is exactly the same, Apple doesn't store the goods, Apple doesn't distribute the goods, Apple doesn't publicize the goods. If Apple forces using their payment processor will be exactly the same in both cases.