To be fair, some of the post-9/11 policies did have an impact. However, don't discount the real effects the attack itself had, before the knee even had a chance to jerk.

I was at Sabre (spin-off of American Airlines, owner of Travelocity, creator of travel agency software) at the time, in HR's development group. (Actually that group had been acquired by EDS, but same group, same role, different name on the paycheck) Within a trading day of the attack, they lost 50% in market cap. We went from massive projects to calculate performance based compensation to doing it on a simple spreadsheet. Many lost their jobs, including me.