I think the argument would be fairly straightforward: the App Store is the exclusive avenue to distribute paid applications to iPhone users. In so doing, it acts as a single seller of applications to consumers and a single buyer of applications from developers, and its arbitrarily-set fees are incorporated into prices.

The fine parsing comes in whether "iPhone users" are a sufficiently distinct market to effectively make the App Store a monopoly, and I think reasonable people could have different opinions here.

If my product is being sold in any retail store e.g. Best Buy then I typically don't have access to the customer's contact details that may be collected at the checkout. It's up to me to capture that information when a user uses my product.