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.