I think it's useful to view these laws as a way to get around the standard of "beyond a reasonable doubt." The actual societal problem -- the one that's been i

I think it's useful to view these laws as a way to get around the standard of "beyond a reasonable doubt." The actual societal problem -- the one that's been illegal forever -- is robbery. But it's hard to catch robbers. If they aren't caught in the first few minutes, there's a good chance they get away for good. When the police stop somebody and find a knife on them, they might think "this person is probably using this to stick people up." The legislature can't pass a law that says "conviction for robbery requires now requires only probable cause", but they can try to achieve the same effect by making it illegal to carry a knife.