As we know from hacking DRM and many other things, there is no 100% secure system. There is only a question how much effort someone puts in succeeding to break it.
I guess any successful attempt from keeping idiots from blowing up an airplane is worth the money, specially because those who spend the money want to avoid the embarrassment to have someone to do something, and afterwards all the people who saw this person say "We all saw him. He was very suspicious."
Of course this is not necessarily effective. But it's the same security why we ask people to present a CV. If the guy you hired screws up big time, you can still say "But he had a PhD from Harvard!".
Of course all of this does not help you spot that someone is incapable/evil/criminal , but you have an excuse that you did what you could to prevent whatever bad happened.