Laws regarding blades and blade length in the United States can easily vary from one state to another. In the sometimes chaotic and confusing world of blade laws, throwing knives may be a very gray area. In the state of California for instance these knives aren't strictly illegal. You can have one but you can't buy one.
According to sosamartialarts.com, a website that deals in various martial arts equipment, it is illegal to sell throwing knives in California. This means that the items can't be mailed to residents in the state, and the knives can't be sold within the state. There isn't anything in the law that states throwing knives cannot be owned by residents (as codes in other states say, naming throwing knives specifically as banned), but being unable to buy them in California does make it more difficult to acquire one.
Throwing knives are not meant to be worn openly. Throwing knives are often specifically designed to be hidden in the sleeve or on the belt beneath a shirt of jacket. California prohibits carrying any sort of concealed knife, regardless of type or length of the blade. This law applies to knives that are mocked up to look like different objects including lip stick cases and sword canes.
The actual lettering of the California law is that anything which can readily be used as a stabbing weapon is illegal, but according to donath.org, this interpretation may vary. Since a throwing knife is meant for throwing and not stabbing, the interpretation might be different. The site goes on to state that in some circumstances, pens have been illegal in certain situations in the state of California because they could readily be used as stabbing weapons.