ambiguity

Way back when, the FSF had computers you could get accounts on, which had Emacs installed. You could get the compiled Emacs executable, and you could get the source and compile it yourself, but you were unable to install a modified version on the FSF's computer (in /usr/bin) because you didn't have the root password. Presumably, they weren't violating the GPL. The difference in the Vizio case is that it's (supposedly) your hardware that you don't have the ability to write to, rather than the hardware belonging to someone else, but the GPL doesn't have any way to distinguish someone who owns the device from anyone else the firmware was distributed to.