I have a 1 1/2" pocket knife I like to carry. It is no more dangerous than a sharpened pencil. I could stab someone, but I could do that on the ground or anywhere.

With hardened, locked cockpit doors, and non-compliant flight crews, how exactly do you think my knife is dangerous to the plane?

When rules are arbitrary and clash with common sense, they are more difficult to remember to follow than rules that do make sense.