If there is anything these secret proprietary built in hardware backdoors are its that they are very poorly thought out.

It does seem to be a pattern. I can think of a few reasons off the top of my head:

1. Incompetence - The engineers try to get it right, but fail due to skill, budget or time constraints.

2. Malice - This allows bad actors to compromise the PSP for evil

3. Benevolence - This allows hardware owners to alter the PSP for their own protection.

I think the smart money is on #1, but they're all interesting to think about.

ARM is the designer of the PSP, if its incompetence then they're the ones at fault.

AMD states that the PSP is "ARM TrustZone ... Industry standard" https://www.amd.com/en/technologies/security

That's supposed for data-only modules, so they can patch in live data later, without the need to fetch it from some flash or bios. I think no-one should call code from it.