It wasn’t their choice to make. The user purposefully installed the app from a 3rd party store. That sounds like user intent. If Apple cared about their users, they would allow a user to use without caveat. Including installing whatever software they wish so long as it worked on the platform.

This is right to repair. This is ownership. When you buy some hardware, you should be allowed to install any software you wish, provided it works and you have the technical know how to do so.