The App Store is not a Wal-Mart. Wal-Mart doesn't let people use its store to give stuff away for free, for example, while Apple has no problem with free apps that never require payment for anything. Wal-Mart also isn't the sole source for products which work with another kind of product, they're just a store, little different from other big stores. The App Store is the sole source for software that runs on iPhones, and it exists mainly to drive iPhone sales, the revenue from the App Store itself is insignificant.
Apple requires all developers to pay $99/yr to publish any applications, even free ones. They are strictly pay to play. Also, App Store revenue is certainly not "insignificant", we're talking $Billions of dollars for Apple here... https://techcrunch.com/2014/01/07/apple-10b-in-app-store-sal...
Good point on the $99/year. Still, good luck convincing Wal-Mart to let you user their stores as a distribution point for your free stuff for a low annual payment.
Apple is big enough that billions can easily be insignificant. According to that article, they've paid out $15 billion total to developers. That means that Apple's cut has been about $6.5 billion. That's over a period of eight years, so somewhat less than $1 billion in revenue per year. Currently Apple is making $10-20 billion per quarter in profit, with yearly revenues beyond $200 billion.
