Comment 82 for bug 1396379

Grub gets rewritten, with a dependency upon the target's root, so if the target is a removable USB, and you remove it, (think a 32GB USB stick you want to give to a friend), grub files get removed too, and your host system no longer boots. (and of course your target USB does not boot either) That's a critical bug according to https://wiki.ubuntu.com/Bugs/Importance:

Critical: A bug that has a severe impact on a large portion of Ubuntu users

    Causes data corruption
    Crashes the entire operating system
        For example, if the system fails to boot, or X fails to start, on various makes and models of computer
    Renders the system temporarily or permanently unusable

It's a quibble to say the bug didn't clobbered the system when the user removes the removable target, after the installer ignores the explicit grub target location.