I really do not know why knives are illegal, or why they stopped him, I am still wondering what a blind man needs with a gun. It’s his right of course, but I wouldn’t want to be too close when he pops a cap.
Airline (FAA) rules have been anything (fixed blade or folder) over 4" is verboten. That's one reason there are a lot of folding knives with 3 7/8" blades.
Blades sharpened along both edges (so called "daggers") are generally asking for trouble. Serrated or semi-serrated edges also get a closer look. That is true anywhere, not just an airport.
Back in the 60s congress passed legislation banning the importation of "switchblades" (Ie push button opening) folding knives. Almost all states still ban switchblades to this day with exceptions for military and police, although you can buy a manual opening stiletto and the parts to convert it to an "automatic" if you know where to shop.
Another "end run" is the assisted opening action which requires you to start the blade moving a bit and then a spring finishes snapping it fully open. Not true switchblades they are generally approved, depending on the blade length, under 4" being the criteria.
There are places where any folding knife that you can open with one hand is suspect and probably illegal. For sure Balisong (butterfly) knives (favored by Hispanic and Asian gangs) where the blade is enclosed within a two piece grip that may be opened with some highly impressive gyrations of a single hand are going to be trouble.
The topic is highly subjective and more often then not comes down to "because I say so". If a DHS screener spots something he fancies, he's going to take it and tell you it's banned. End of the day, he goes home with it in his pocket. What are you going to do about it?
I have carried a knife since grade school and feel naked without one in my pocket. For that reason, I avoid airports and any other place with metal scanners/x-ray machines. The last time I flew anywhere was Feb 1992, pre-DSH, so I carried a small flat folder with a clip. I stuffed my folding money under the clip and the screener passed it off as a money clip. When I flew back I brought in two switchblades that I had purchased in Germany. They were packed in my checked bag so no problem. My boss got busted for bringing in some tinned sausages. Hee hee!
Hope that clears things up a bit.
Regards,
GtG