I really don't know the helmet law in California. I do know that too many of us have gone down (even me) and it's really easy to hit your head on the pavement when you go down at 25+ mph.
All of us here in Nor Cal...except Big John haha wear full face helmets. Plus we all know that wearing a helmet reduces the chances of seriously getting injuried. Bones will eventually heal or can be replaced but its hard to recover from a head trauma.