I think we're at a point where we simply disagree, but I do want to throw in my last two cents (and I'll even let you have the last word if you want).
I don't think this is Apple's way of trying to prevent people from making money. On the contrary, I think it's about both parties making money. However, it's most importantly about Apple making money, and scarily, it's about Apple making all of the profit. Receiving a 30% cut of anything beyond an in-game, all-digital good is almost certainly going to eat up the profit margin for any real business.
And all of that just so that you can provide the same experience as a company's existing website? As a business, I think this is Apple abusing its position, but I do not believe that Apple qualifies as a monopoly (so no one can slap them for it). I think my point is driven home by the fact that Apple does not even let apps link to a website allowing users to purchase something outside of the Apple system, nor can they even reference how to do it within their app. With that in mind, I would say that that provides an incredibly broken system when a company refuses to give Apple a 30% cut, or when a company simply cannot afford to do it even if they wanted to join. The app loses expected functionality, and it creates a poor experience across the board.
It is Apple's App Store, and therefore they do make the rules about what gets hosted. But this boils purely down to greed. Apple should provide such an API, and I still think they are free to take whatever percentage cut (although I still think 30% is ridiculous at best), but they should also provide an API that allows the collection of payment information in a consistent, and similarly compelling manner. Considering how generic of a process the collection of a credit card is, they could easily do it. The only reason that Apple refuses to do it is pure greed while protecting themselves from any-and-all competition at the same time considering that both PayPal and Google Checkout could both swoop in and provide a much more attractive model that is also consistent for users across apps, while actually enabling a company to make a profit from a sale.
And I do not want you to get the impression that I hate Apple. I have owned multiple iPhones, and I own a Sandy Bridge-based MacBook Pro (which is still the current generation, for now). I love their hardware quality, as well as their support, and I even like the ecosystem of apps. Even with that, I still think that this is overstepping by a large and unreasonable amount (referencing the idea that you must go through them, not even the amount that they take).