If someone wants to kill a few hundred people at an airport, surely the queue for security is the place to do it. Stand in the middle of the meandering lanes of closely packed punters and kapow! or is it kerpow?
So we need a pre security security line to prevent such a thing happening.
Ah, hang on, we need a screen to stop people bringing dangerous things into the airport.
Umm, no luggage allowed, sorry.
Hand luggage, no chance.