It does seem weird to protect user space memory from the kernel but not the memory of kernel subsystems from other kernel subsystems. But at that point we're a

It does seem weird to protect user space memory from the kernel but not the memory of kernel subsystems from other kernel subsystems. But at that point we're at a microkernel design that needs context switches between kernel processes... Good, I'd welcome that. This feature just implements a level of security that the whole kernel needs to catch up to before it makes sense.