Your local monopoly of water provider charges a fee not a tax (although many collect taxes on behalf of the state that sanctions the monopoly)
Platform apps stores tend to be a monopoly (specially in iOS case) and that decreases incentive to provide a decent service/product to their customers who hate that fact - not because customer have no representation.
I never submitted an app to play store but knowing how Google handles app customers (or any customer of their services/products) its believable that app devs are shafted too. I personally wonder if there's any differentiated service level between an app that results in revenue for Google vs one that is 100% free. I'd be pissed if my 15% fee is not buying me priority over the latest adware filled flashlight app