They may be able to get around that by making the bricking "accidental" (e.g. "we didn't test the firmware upgrade on hacked consoles" wink wink). But in general Nintendo shouldn't threaten bricking even if they're not planning to do it.
Scarily enough, a few years ago, people were out pushing a bill (which got dubbed the "right to virus") that would have made this sort of thing legal. And to our PCs, too.
