Suppose you produce automobiles and offer the purchaser the choice of different colors. Firebird Red is popular right now, so you order a hundred gallons of it. Will it be enough? That depends on its continuing popularity. If it is popular next week, why, we'll order another hundred gallons. They will arrive "just in time" to apply to the incremental car ordered in Firebird Red.
Except that the paint won't now arrive just in time if the truck transporting it is held up on the Canadian border for six hours. And it will be more expensive, because the truck driver's efficiency has been cut by one-third, as he sits in an immobile truck, waiting in line.
What does this tell us about the theory of security practices? What it tells us right away, and easily, is that the cost of security extends beyond the cost of the people hired to look into trucks crossing the border, searching for explosives or vats of poison. Does this inform us as regards security policy? Of course it does, because reasonable compromises need to be made. If we subjected everybody who wanted to fly somewhere to a strip search, we could be very confident that no weapons got aboard, and pretty confident that no passengers would want to fly. What is a reasonable level of security?
I think back to 1964, to J. Edgar Hoover appearing before the Warren Commission investigating the assassination of President Kennedy. Kennedy was shot by Lee Harvey Oswald. It was known that Oswald had been a Soviet sympathizer who had considered immigrating to Russia. Why hadn't Hoover, anticipating the journey to Dallas of Mr. Kennedy, detained Oswald, keeping him away from the presidential itinerary?
Hoover replied that if he had had to search out everyone in Dallas whose dependability was at the problematical level of Oswald's, he'd have had to round up 150 people. If the president were going to Chicago, Mr. Hoover said (the numbers here are from memory), he'd have needed to detain 450 people -- and "the American people wouldn't put up with it."