I believe helmets have been compulsory in many (most? all?) U.S. racing events and large organized rides for many years. If race organizers can mandate equipment selection and contestant qualifications and behaviours, they certainly can decide whether to require helmets.

However, for transportation and recreational adult cyclists in most states of the U.S., helmet use remains, as it should be, optional but strongly recommended. I support California's current law, which requires helmets for cyclists and 'bladers under 18. I forget where I first read this, but it seems appropos: "It's one thing for daddy not to wear a helmet because he's an idiot. It's another for the kid to grow up an idiot because he didn't wear a helmet."