Well, one could easily argue that knives and box cutters do not pose a threat to planes (they certainly don't today--there's no way a would-be terrorist could use a knife to hijack a plane now). I know a lot of people who carried small folding serrated knives in case they had to cut away their seatbelts, etc., in an emergency. So that the SOP in place allowed knives on board is not an issue.

You might start to place blame on the lack of reinforced cockpit doors, but the truest finger to point should aim at the policy of complying with hijackers. Flight crews at the time believed the best way to avoid loss of life was to cooperate with any hijackers (much like the idiotic instruction to not resist a mugger or rapist should you have the means, etc.). No one expected the planes to become guided missiles, but the policy of cooperating with hijackers is surely one of the chief reasons 9/11 unfolded the way it did with 3 of the 4 planes (by the 4th, the pax had caught on and took actions that resulted in the hijackers aborting their mission)

Nothing is "normal" anymore. We have this fear-driven acceptance by the public at large of a complete and total abridgement of our rights, intrusive yet arguably ineffective security measures in place whilst leaving open many more legitimate gaps in security, etc. The airlines have been dealt crippling blows by the terrorism that they have never recovered from, and many employees of the travel and tourism industry lost their jobs after 9/11 due to everyone wanting to go stick their heads in the sand.

What we have forgotten are the lessons of history, of how we won the freedoms we have (or "had," in some cases).