Leaving aside the fact it's rarely the case you can just sign away liability..
The GPL only applies if you decide to accept its conditions. Just installing Ubuntu doesn't mean you've agreed to the GPL and, as such, Canonical has anything to point at if your Nuclear Reactor has a meltdown because a bug in Unity swapped the "Drop fuel rods/Raise fuel rods" buttons by accident.
Sure, you might give up your right to sue if you subsequently redistribute Ubuntu to others. But even then... like I said, it's rare you can just sign away liability.