I think it's funny that in most of America, someone with a permit can carry a brace of automatic pistols and enough extra magazines to mow through a dozen preschool classes; but brass knuckles and spring-opening knives are still off-limits.
Many laws in the US regarding weapons were motivated by racial and economics tensions, as in people were worried about disenfranchised groups acquiring weapons and revolting. Maybe that has something to do with it?
> automatic pistols
Automatic guns are extremely expensive and hard to get because none manufactured after 1986 can be sold to civilians. They are rarely used in crimes. Automatic weapons are not magic killing machines. They are excellent ways to turn money into noise but have limited tactical use, suppression fire, etc.
I think your first guess is correct. Machine guns are owned by rich enthusiasts, brass knuckles are owned by poor thugs. It's implicitly discriminatory, although any rich switchblade enthusiasts are simply out of luck.
Come to America. Open/concealed carry is legal for every sane and non-felonious adult in many areas, gentleman and commoner alike.