I brought this concept up a long time ago and I got baked on here for it. Everyone on the plane has a right to be secure against attacks. This means that even though the whole plane was not used as a weapon when the passenger in 3a pulled a knife and started slashing other people, those other people could have been protected. You cant let some things go on a premise that the passenger beware. One thing that I have learned is that people DONT read signs, DONT listen to announcements, DONT watch video presentations etc.