But even if they were only taking a 5% cut, I think the argument still stands that they have a negative impact on consumers in that they decide what the hardware you bought is and isn't allowed to do. Apple can invalidate entire business models if they disallow an app on their store, and an entrepreneur can spend millions developing a business only to have it rendered inviable by a change to the terms and services which can be made over night without warning.
All of this could be solved by allowing side-loading. It wouldn't even have to be made easy to do - go ahead and bury it deep in the settings behind a bunch of scary warnings. But I see no excuse for not allowing this at all.
At the same time, those 'app store' are not exclusive, they allow 3rd party 'app lists' or at least allow other means to distribute 'apps'.