I agree. Charging a blanket percentage of gross revenue is an extremely inexact way to monetize what is a broad basket of services that were previously separate including: electronic software delivery, software security verification, marketplace, transaction processing, DRM, etc. Since 2009, first on Apple's app store and then Google's, these services have all been arbitrarily bundled together despite having vastly different one-time, fixed and variable costs. People are only used to it in this context where every marketplace has been controlled by a monopolist gatekeeper.

Doing it this way makes no economic sense for either the seller or the buyer but it's coincidentally the absolute best way for a middleman to maximize the tax they can extract from a two-sided marketplace they control. In competitive markets, blanket taxing on total gross revenue generally only occurs when there's a single fundamental cost structure tied to that revenue, or the amounts being collected are so small it's de minimis. App stores are highly profitable, multi-billion dollar businesses.

Perhaps the most perverse thing about this is that electronic transactions for purely digital goods which occur entirely on real-time connected digital platforms make it trivial to price each service for maximum efficiency. It's easy for the price a 2GB game with frequent updates pays for electronic delivery to reflect the cost they impose on the infrastructure while a 100k one-time purchase app can pay a vastly smaller amount. And that's exactly the way the competitive marketplaces evolve - from moving shipping containers around the planet to residential propane delivery.