And they would have to get together, and mix them, and the passengers would know what's up. Passengers have stopped the shoe bomber, the underwear bomber, and a few other incidents of rowdy passengers though not terrorists. In fact all terror acts in flight have been stopped by the passengers since 9/11. I think the idea is, sure, they could get enough of it on board, but they will stand out like sore thumbs and have to get together to mix it or whatever. People would step in. It prevents one or two people from being able to carry it out, and now requires a much larger group of people. It's a deterrent.Think about the 3 oz. rule. Let's say that the kind of liquid explosive that the TSA uses to justify this rule is an option that terrorists would choose. Well, given that many planes are capable of carrying hundreds of passengers, could a large enough group of passengers not simply carry all the material needed to construct a device secreted amongst them?