Well, you're right that grub-the-bootloader doesn't have a supervisor to stop it from writing to random places on the disk (since it's not running under an OS),

Well, you're right that grub-the-bootloader doesn't have a supervisor to stop it from writing to random places on the disk (since it's not running under an OS), but grub-the-bootloader doesn't actually write to those sectors, so I'm not sure what your point is. grub-the-comman-line-utility writes to those sectors but it runs in linux userspace so it's really in the same position as the windows programs that also write to the reserved space...