A lot of people have nothing to gain, and everything to lose by opening their mouths. Nurses gain nothing for spouting off, and words can be tracked down to the person fairly easily, even before the internet.

It is fairly easy to intimidate people into capitulation if their supposed lifeline (HR, security, etc) side with the boss.

If you think about it too, these ARE bad guys. One 'accident' could shut up a whole lot of employees.