The premise of a walled garden is to keep unwanted things out, not to imprison you inside. Apple maintaining a store where they've vetted everything in it is fine, and if you like you can refuse to install anything from outside of it.

That doesn't justify them prohibiting you from installing anything from outside of it. It should be up to you.

If you wanted to, you could even configure your phone to not add any new stores without a factory wipe. But maybe first you want to add in the repositories that have only free and open source software, or the stores of some respected game publishers who offer lower prices if you use their own stores for their games. And maybe the existence of these stores would encourage Apple to charge lower fees, and then you benefit from the lower fees even if you choose never to install anything from those stores, since your option to exerts competitive pressure on the stores(s) you are willing to use.