The logical conclusion of applying this rule evenly is a scenario where you can't install apps except via the Play store, even if third party app installation is technically "allowed" on paper, because you can't actually install an app that will install apps. This is (yet another) stupid decision by Google, and Hanlon's Razor might not apply since they have a vested interest in the Play store being the only place to get Android apps.

"Of course the OS allows you to install third-party apps. We totally respect your freedom! Now, you'll have to install those with a third party installer, which aren't allowed on the Play store. And any app that would allow you to download and install the third party installer also isn't allowed."

A file manager which opens files should be allowed to open APK files.