ChrisVJ,
You seem to have missed the point.
The USA already had a locked door/restricted access policy that had been in effect for around 15-20 years.
In all that time there have been only a few cases where approved access to the cockpit was used for nefarious purposes.
The locked door policy didn't stop the 11/9 hijackers, none of whom were part of the 'approved access' crowd. Nor could it.
Up until then guidelines & recommendations ALL advised cooperation with hijackers, with the intent of getting the aircraft on the ground where negotiations could take place. After all, the a/c's time in the air is finite.
The 11/9 hijackers changed the 'accepted' norm.
They were able to assume control of the a/c because current policy specified that cooperation is appropriate.
It matters not whether they boarded with box cutters or whatever. Just about anything would have served as a weapon. Glass bottles (think how many are on board or are available by the thousands at the duty free), an innocuous strip of metal eg those often found in spring steel clips or on folders, even a sharp pencil or fountain pen would have sufficed to slice someone's throat open. Shall I mention safety razors? Or karate?
All that was needed was a means of threat + demonstration of willingness to proceed with the threat ie slice the throat of the nearest flight attendant or passenger, and cooperation that includes access to the flight deck was assured.
It was the then policy of cooperation that was instrumental in their gaining control, not the discression about who is allowed up front.
Of all the thousands upon thousands of flight deck visitors or jump seat rides how many resulted in 11/9 events? None.
Meanwhile we have the ludicrous situation of flight & cabin crew being searched for inane items are deemed to be offensive weapons or being refused access to the jumpseat, conveniently forgetting that we already have access to the flight deck just by turning up for work.
Idiot rules devised by bureaucratic idiots in the name of some laudable goal doesn't excuse the idiots.