Legality of Bicycle Courier Helmet Use in California

I've mostly tried to stay out of this because there is so much emotion involved with this issue you can't win no matter what you say. However, someone called expressing concern over some Clutch Couriers not wearing helmets 100% of the time, and threatened to "report me to the authorities" for not supplying helmets to my riders.
All I can say is that I recommend helmet use, especially in wet or high speed conditions, and that I try to set an example by wearing a helmet every time I work. Also there is a brand new Specialized helmet ($175) in the office for anyone who wants to use it, however, YOU CANNOT LEGALLY MANDATE HELMET USE FOR ANYONE OVER EIGHTEEN IN THE STATE OF CALIFORNIA.
In this state it is an adult choice whether or not to wear a helmet while cycling and I encourage anyone driving around without a helmet to investigate the top causes of severe head injury (motor vehicle collision) before screaming out your car window to "get a f'ing helmet."

Ride Safe-RG