I start by saying that the closed cockpit door rules probably would have come about anyway BUT,
911 was a bunch of guys hijacking a bunch of planes with a bunch of box cutters.
Prior to that date it WAS LEGAL in the USA and many other countries to carry knives/box cutters on board as long as the blade was no more than 4 inches long.
911...whose fault?
You decide.
Sorry kids, you can't visit the cockpit any more.