Honestly it could be that developing and maintaining these region-locked differences in OS might be more expensive than saving every last penny from not allowing piracy (which is the real deal for this fuss).
Big majority of android users don't sideload either, most people are so technically illiterate they don't really grasp the idea of an App Store overall, it's just a place for them the get an Instagram button on a new device
Eh, I know plenty of developers glued to the apple ecosystem who could probably have a lot of fun with it if they were able to. They just don't rate it as important enough to counteract the things they like about the ecosystem.
I doubt that. There's already a lot of other features out there that have to be region locked because of local laws. Could be medical related, car crash detection, etc. The cost of an engineer just reusing that same logic would be fairly minimal.