Well, I'm not a lawyer but I would argue that they have no legally justifiable reason to do so - their function is to prevent you from taking weapons aboard the aircraft, and if you take the knife and put it back in your car, they've done that.

Of course, as I understand it, the US Supreme Court has pretty much made a mockery of whole "protection against unreasonable search and seizure" thing, but I'd be curious to know if anyone has challenged this in court?