And all these devices certainly should be able to run the code their owner wants them to run, full stop.

That this is often not possible is stupid but also incredibly dangerous. Not by the standards of the evil Apple just did, but what even more malicious actors can do to you is they can run code on your device & you can't do anything about it.

It could be state actors pressuring the company making the think & or exploiting vulnerabilities in the closed source firmware running on it.