That's the strange thing about the No Fly list. If the people are dangerous and they find out they can't fly (which could really only be because they were on the No Fly list) then wouldn't that make them more likely to do something. Hell, it's the sort of thing that is going to anger someone who wasn't dangerous and had no inclinations to hurt anyone. Sure, they likely still won't hurt anyone but suddenly protesting and fighting against the power of the government doesn't seem that outrageous when the govt. is clearly fighting against them.
Also, if the government truly is afraid of an actual threat by an individual don't they have an entirely separate list (like the BOLOs) that tells law enforcement officers to not only watch out for these individuals but apprehend them.
What's the point of a no-fly list when the individual can travel by other means to another country and then fly on to where ever they wish. Sure they can't fly into or out of a US destination but isn't the rest of the world open to them? It seems the only thing a no-fly list is really for is to stop someone from taking over a plane and if someone is really at risk of doing that shouldn't they be in jail or at least under constant watch so a No Fly list isn't need?