I am 100% OK with Google, Apple, or whoever charging whatever price they want to charge for using their app store services. They are, after all, providing all of the infrastucture for software delivery, updates, payments, etc. Not to mention giving you access to a massive potential userbase.
The problem is them preventing you (the developer) from allowing the user to use other forms of payment processing within the app itself. Apple is the worst offender of this. Google obviously allows side-loading of apps and other app stores for Android while Apple does not. Apple goes so far as to prevent you from linking to a website with an external payment option, or to even suggest in text within your app that there are ways to subscribe/pay outside of the app store.
They claim it's about the user experience and security, which is legitimate, but it's almost certainly about keeping those nice profit margins. It's within their right to charge what they want to use their store, but the restrictive nature of not allowing competition is wrong. If Apple would even allow sideloading of apps this wouldn't really be an issue. They could keep things as-is and just tell developers "Don't like the app store rules? That's fine, but you can't list your app here. Good luck in the free market."
Because mobile computing has become the primary form of computing for many (most?) people, the fact that these few companies have so much power about what people can and can not do on their own devices is scary.
Edit: At a minimum, they could require developers to make all payments available via the official app store payment platform, in addition to any other types of payment processing they want to do. This would let the customer decide and would get rid of any regulatory concerns about monopolies. And you know what? I bet most customers would still pay via the official app store/play store payment method, but at least the other options are there. I also think the policy of not allowing apps from outside the app store to be installed is insane. It's your device, you should be able to use it how you want to.