Comment 12 for bug 1273764

This appears to be by design, to prevent people setting it to 0 and not knowing that you can hit shift (or escape if that fails) to get the menu back.

On a Mac and possibly other EFI based systems this is undesirable behaviour as I can already choose what OS to boot from the firmware, in my case by holding ALT, so all this does is add more delays and flickering screens to the boot process.

I love Ubuntu for its tendency to "just work" (much like the Mac's native OS) but I do not need to be babysat, and really must object to cases like this where "nanny scripts" actively fight against me making customisations to my system.

I recommend adding a comment to /etc/default/grub warning users of the 'dangers' of setting it to 0, instead of fighting them. Those who want to tinker with things and not understand what they're doing will always find ways to hose their systems, but most of them are grown adults (if not they are poorly supervised kids) and can take responsibility for their own actions and even learn something from their mistakes. All this does is cause confusion for those who do know what they're doing and work with GRUB in other distros.