It is interesting that it is codified in the Title 15, Commerce and Trade, section of the U.S.C. and not in the criminal code, Title 18 U.S.C.
And it isn't an affirmative defense; it is simply a non-applicability. An affirmative defense is brought up to the court once charges have already been placed in court.
Personally, I would agree with you in your assessment of bothering a disabled person carrying such a knife.