In California, unlicensed ownership of weapons resembling assault rifles is a felony (See <http://www.leginfo.ca.gov/cgi-bin/waisgate?WAISdocID=35675128432+5+0+0&WAISaction=retrieve>). This law was extremely controversial.
The Association of California Cities, a prominent supporter, said that California law enforcement agencies feared that some groups in large cities might undertake successful rebellions against civil order if armed with modern weapons. Prominent members of black churches in L.A., as well as Senators Boxer (D-CA) and Feinstein (D-CA), have said that the only real purpose of such weapons is to kill people, and therefore there is no reason to permit them.
In contravention to some claims of both the above anti-gun forces and the California Rifle and Pistol Association (CRPA), the California legislature clearly permitted conventional semi-automatic rifles and ammunition, including military calibres and velocities, especially on private property. However, possession of fully automatic weapons is felony in California, unless a state license is procured. The licensed exception may be a concession to the state's entertainment industry.
This legislature was clearly opposed to disarming the state's citizens, yet clearly opposed to private ownership of military weapons. Interestingly, Californians retain rights to sufficient types of arms for practical self-defense, and possibly even rebellion.
Members of the California Rifle and Pistol Association see civilian style "assault weapons" as collectors' items, or realistic training aids for civilian enthusiasts. Some have said that they see them as useful for civil defense in times of civil disorder or invasion.
Based on engineering differences, the ease of modification, and their high level of expertise, CRPA members see nothing special about civilian "assault rifles" except their appearance... which is exactly what the collectors and civl defense enthusiasts want.
In summary, CRPA members say that a ban based on appearance is faulty. The California legislature chose to outlaw threatening weapons.