Most hardware vendors that implement this sort of functionality add prominent warnings to the lowest level boot screens notifying the user the device is "unlock

Most hardware vendors that implement this sort of functionality add prominent warnings to the lowest level boot screens notifying the user the device is "unlocked" or in "dev mode" or some such. The idea is that the absolute lowest levels of firmware remain signed and controlled, while giving the option of loading unsigned code on top. So in theory, you get the best of both worlds. I don't think there's anything really stopping Apple from doing this, but as many others have said, it's not really their jam ;)