They probably know that.. it's probably just a threat to discourage anyone stupid enough to try it anyways. The problem is.. they are already stupid enough to try it anyways.
However I still say in this day and age there isn't going to be a plane anywhere in the world that is going to be taken hostage today without some passengers putting up one hell of a fight. They better be armed with alot more than boxcutters.
This strikes home because I will be on one of those planes. And for the record, no matter what they are armed with, past passenger conduct after 9/11 indicates passengers will beat the feces out of anyone who tries something.