It is unfairly funded. The ensuring part requires the same time and effort for
Free apps, apps that make money off ads, apps that cost $1, apps that charge a monthly fee, apps that resell products where a 30% cut exceeds their margin.
Perhaps the stores should change to a model of choose your own adventure: pay per release or % cut. What would you say is a fair amount to charge per release - keeping in mind apple/google will want to maintain their insanely high profits from the app stores?
"Ensuring that Kindle doesn't break my smartphone" requires time and effort. That's funded by the cut they take.