This is what I thought, no one can find such a law in "most states."
Kind of like the people who claim that it is illegal to own throwing stars,
Perhaps those people are in CA.
Both batons and throwings stars are criminal offenses in CA. Batons are covered under California's legal definition (several of which are not the dictionary definition) of "billy".
Throwing stars are specificly outlawed.
In CA carrying any object for use as a blunt weapon is a crime, and can be charged as a felony. Whether it is a bat, a lock on a bicycle chain (case law specificly deals with such an incident that creates precedent) a stick, or even a perfectly legal object intended for use as a weapon.
Items with no purpose other than a blunt weapon are automaticly a crime.
However legal items carried for use as a weapon must be proven to have been carried for use as a weapon (and saying you carry it for self defense, or could use it for defense is the most common admission of guilt to carrying it as a potential weapon.)
It does not have to be used in any crime or be intended to be used in any crime. Just possession is a felony.
The legal definition of sap, slungshot, sandclub, or billy is very different than the dictionary meaning or even the meaning of those terms in some other state laws. It also includes "leaded cane" which does not mean "leaded" cane but any object made heavier to strike better.
The specific definition given in the penal code and backed by case law covers any object intended to be used as a blunt weapon. Sandclub and sap are defined as practicly anything heavy, or weighted for use as a weapon.
A purse made heavier with change for example qualifies as a sap or sandclub if intentional done for use as a weapon.
I would copy the legal definition here, but it is long, and then I would also have to cite relevant case law, which is tiresome and long even in summary.
However to keep it simple, any blunt item clearly meant to be or designed as a weapon is automaticly a crime.
Any regular item not designed as a weapon (like a flashlight, bat, chain, roll of coins, rock etc) is a crime if the person intends to use it as a weapon. Even if you say you carry the flashlight to see in the dark, and in case someone attacks you, you just admited to a felony. Ever even mentioning its use for defense or as a weapon makes it a crime.
Much of it but not all is covered under the laws in PC12020.
Ironicly it is legal to keep an axe intended to be used as a weapon. Or a sword or massive knife intended to be used as a weapon (open carry only to avoid violation of "dirk law".) But carrying a flashlight for use in self defense would be a felony.
Why do you have that sword on your waist? "To cut anyone that attacks me in half" would be a perfectly legal response (though not wise.)
The law is the law, when you start thinking it makes logical sense and you don't need the details you will get into trouble with it.