"Ensuring that Kindle doesn't break my smartphone" requires time and effort. That's funded by the cut they take.
Apple / Google do not need to take a 30% cut of every e-book ever sold to fund their review. That's absurd and illogical.
One estimate is that Amazon sells 487,000,000 ebooks a year. If we assume they cost $10 each, that's almost a $1.5 billion cut for Apple and Google.
It does not cost $1.5 billion for Apple and Google to review Kindle to make sure it doesn't damage devices.
What if this is a service I don't? Why am I paying for it? The vendors selling in app items are certainly passing on some of the cost of this.
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?