What on Earth makes anyone think that a plane can still be hijacked with a knife or box-cutter? The passengers would NEVER permit it! They would know in no uncertain terms that either the hijacker would kill them by crashing the plane or the US military would. As soon as someone on the ground finds out the plane has been hijacked - within a very few minutes a squadron of US fighters would be screaming across the skies at Mach whatever to bring down the plane before the hijacker brings it down himself.
Looking for knives and box-cutters is ridiculous and a waste of time. Better to look for a middle eastern appearing man wearing Herman Munster shoes with a bomb wick sticking out of them and confiscate his cigarette lighter.