Wait, what? The markup at a supermarket has NOTHING to do with farmers. Markup at a supermarket covers their expenses (location, staff, utilities, equipment, etc.) and their profit margin. Farmers etc. account for their cost of goods.
Stop acting like the iOS app store is a "single web page" and nothing else. Like any other retailer Apple builds and maintains the store which has costs associated with it like physical workspaces, staff, equipment, servers, etc. and they deserve to make a profit.
Most apps are FREE meaning the only way to make any money from the distribution of that app is through a commission on IAP. Without a commission on IAP how exactly should Apple cover their costs and make a profit?