As a developer myself, I am shocked how people treating Apps they "paid for".
When we "sell" our software, we sell the "right to use", we don't sell the "ownership" unless explicitly intended to.
In other words, we developers own the apps, not the user, and we will write the copyright documents inside the apps.
If we don't want to include "right to use" on a specific platform in a sale, the we should have the right to.
But I personally is NOT against the sideloading approach even as a developer: I will not encourage that, but I will not implement blocking mechanisms either (it is easy to detect M1 devices and make your app not run).
The reason behind Apple doing such might come from the complaints of major developers.