Your cereal aisle analogy is a bad one, for two reasons. For one, everything about the merchandising of a particular product in a big box store is agreed upon

Your cereal aisle analogy is a bad one, for two reasons. For one, everything about the merchandising of a particular product in a big box store is agreed upon in advance between the retailer and the manufacturer before the product is placed in the store: the product placement on the shelves; what kind of manufacturer incentives (e.g. coupons) the retailer will honor for the product and how renumerations will be made from the manufacturer to the retailer for them; what kind of sales volume is required to trigger volume discounts, etc. These agreements are highly bespoke in a way that couldn't be scaled with the sort of volume that's on app stores. Second - physical retailers are compensated by the manufacturer when they accept a manufacturer's coupon. The transactions are logged, summarized, and traditionally the physical coupons were collected and sent to the manufacturer. Barring a pre-negotiated agreement to accept less than face value in exchange for a coupon, the retailer isn't losing revenue on the deal. However, when an app publisher offers a means to circumvent the app store payment system, Apple/Google DO in fact face a loss of revenue.