But I'd like to comment on several points raised during this thread.
Yes, the OP's first post merely asked if helments were mandatory in endurance events. However, you also commmented on helmets being uncomfortable, which kicked off a lot of the rest of the conversation. Threads don't drift off topic by themselves. That this thread turned into helmet vs. anti-helmet was as inevitable as the sun rising in the east.
While nobody likes to be lectured to; if you state what you reasonably know is a controversial position on a public message board, don't be surprised if a lecture follows.
"I can't be sued" is a gross oversimplification of the equine liability laws. Anyone can sue anybody for anything. How far they get, and how likely they are to prevail, does depend on local statute and precedent. My understanding is (and please, anyone with actual legal training, please correct me if I'm wrong) that none of the existing state equine liability laws protect you if the plaintiff alleges negligence. Horse spooks, rider falls and gets hurt, no liability. Horse spooks because barn owner drove a tractor by the ring where a beginner lesson is taking place, horses spook, rider falls and gets hurt - you may be liable because you were negligent. A reasonable person could predict that the tractor would spook the horses and beginners couldn't control a spooking horse.
Virginia's law says in part "shall not be liable for an injury to or death of a participant resulting from the *intrinsic* dangers of equine activities" (emphasis mine.) Intrinsic means the unpredicatability inherent in working with horses. It doesn't cover an instructor failing to tighten a girth or cinch, or putting someone on a horse that they reasonably could predict the rider can't control - that's negligence. It doesn't cover event organizers who allow events to continue in unsafe conditions.
Riders and professionals in any state need to understand their own's state statute, and exactly what it does and doesn't cover. It is not the blanket "Get out of jail free" card that people seem to think it is.