As a more extreme example, lets say I have software with a license that requires you to punch yourself in the face in order to use it. If I find you've used it without punching yourself in the face; I can't compel you to punch yourself in the face, I can only collect monetary damages and get a court to forbid you from using my software anymore.
When you've tightly tied yourself to a single GPL product, it may be easier to just comply with the GPL than to switch to using non-GPL software, but a bootloader is hardly a tightly integrated part of an OS; they could release with grub 2 and switch to elflinux if it is determined that they were violating the GPL3 by not divulging their keys.