Back in 1995, when airlines ran pre-boarding screening there was a debate over what is a weapon. It was decided by Transport Canada and the United States that for the interest of looking for explosives and the interest of improving throughput through checkpoints, knives of less than four inches should be allowed. Six years later, all the weapons that were used were legal to bring on board. So a loophole was exploited.
On February 15th, 2011. See this statement in context.