I think we may be reading this wrong. I can't imagine this limit would be put on motorcycles. Compare it to other motorsports- CA is the custom vehicle capital of the world. CA, and every other state, allows- to a certain extent (CA is actually one of the most free)- modifying vehicles. Trucks can be lifted or lowered. Cars and trucks have custom suspension fabricated, sometimes by skilled people, sometimes by back yard butchers. Frames are cut, modified, and welded. Tires fitted which are literally two times the original height that came on the vehicle (and often even larger). And all of this is legal. Custom bike shops start with a pile of metal and fabricate choppers that sometimes barely resemble a motorcycle. Parts are used which were never intended for a motorcycle. Bikes built with engines from other bikes, cars, and even helicopters...all legal. And they set the limit at using a non-motorcycle specific rear tire??? I really find that hard to believe.
I also find it hard to believe that it could pass muster in any court, to allow a private company to impost laws and restrictions on vehicles, without going through a voting process.
Having given it more thought, I really think people are reading the wording wrong. I think the wording simply allows CA legislators to look at outside studies and suggestions before making THEIR OWN LAWS. And if there is no law specifically stating that a non motorcycle tire can not be used on a motorcycle, then enjoy your darkside tire.
Insurance companies are an entirely different situation- they are private companies, and have the right to limit just about anything they want. We are also free to choose a different insurance company if we don't like what they limit.