I think it's doubtful that kubatons are generally illegal in California.
California Penal Code Section 12020 bans possession of, in part: "...any instrument or weapon of the kind commonly known as a blackjack, slungshot, billy, sandclub, sap, or sandbag." I think it would be a stretch to fit a small palm stick such as a kubaton into the definition of "billy," and they are clearly not blackjacks, saps, slungshots, sandclubs, or sandbags.
Section 12020 also bans "metal knuckles", which are roughly defined as something worn on or in the hand which cushions the force of a blow (as to the hands) or concentrates the force (as to the recipient of a strike). The term includes a projection from the hand. Thus, a koppo stick (kubaton with straps allowing it to be worn) is probably a type of illegal "metal knuckles," but a kubotan, which is simply grasped and not worn, probably is not.
Plastic knuckles are also controlled, but only as to the importation for commercial sale or offering for commercial sale (Section 12020.1). So a plastic or wooden koppo stick would probably be OK.
However, many police and sheriff departments have a policy of arresting anyone who tells them an item is carried as a "weapon" (offensive or defensive), and/or confiscating the item. This is probably based on local law, such as Los Angeles Municipal Code Section 55.01, which states:
"SEC. 55.01. CONCEALED WEAPONS PERMIT.
No person, except a peace officer shall wear or in any manner carry
concealed upon his person, any loaded or unloaded gun, pistol or revolver, or any other dangerous or deadly weapon permitted to be carried by law without having, at the same time, actually in his possession, and upon his person an unexpired permit so to do issued by the Board of Police Commissioners."
Violation is a misdemeanor. So for any cities and counties in California that have a similar law (you can search for yourself on the web; try, for example
www.amlegal.com's Online Library), a kubaton might be considered "illegal." But the L.A. law is worded somewhat vaguely and is not a statewide law. Practically-speaking, most citizens caught with a kubaton in a jurisdiction with such a local law would have their kubaton confiscated. But that's not to say all: some would be arrested and possibly charged. I'm using a lot of weasel words such as "probably," because the outcome of an encounter with law enforcement while in possession of a kubaton is uncertain.
I just did a quick seach of California case law on the subject and didn't find anything.
If I've overlooked any law, please respond with specifics.
Regards,
Johnny