The key difference is that if one retailer doesn't have the product that you want then you can buy from a retailer that does.
Apple, on the other hand, actively prevents any other retailer from selling iOS apps.
your logic is flawed. in the physical metaphor, apple owns a store. there are many other stores. those other stores can sell applications like AppGratis if they wish.
but requiring other stores to sell *iOS* apps would be like forcing mcdonald's to allow other stores to sell Big Macs, which is unique to mcdonald's.