These WalMart, Best Buy, etc. analogies are all based on a flawed premise.
Apple does not operate like any of these stores. Once the product is purchased from a store, that is the end of the relationship between the customer and the store.
Say I want to buy a prepaid cell phone from WalMart. I can do that and WalMart gets a cut of that initial sale. From that point on, I can buy minute refills from ANYWHERE, including the carrier directly. I have no obligation to return to WalMart to purchase any refill cards.
In the App Store's case, Apple requires any purchases made after I've already downloaded the game, to be completed through them. Whether app is a paid app or free app is moot.
My relationship with Apple is no longer required once the app is installed on my phone. All the online components are hosted on the developers servers, not Apple's. Apple's requirement to be the middle man for future purchases unrelated to Apple is purely for their own profit and they can do so because they don't allow alternative stores.
If I don't like WalMart's store policies, I can go to another store to do my business. There is no such option for iPhone/iPad users.