Ridiculous. Do they really assume that the majority of helmet-wearing horse people are wearing helmets because they don't trust their horse, or don't have a good enough relationship with their horse, and therefore assume that they are going to get thrown? Quite a lot of people have well mannered, well trained horses but no amount of bonding with any horse is going to guarantee that it won't spook or trip and throw you. That horse could love you but it's still a bloody horse and that's what horses do. My horse -- and many others I have known -- goes out of her way, doing her best to make sure you stay on as that's what she thinks her job is, but that doesn't mean I haven't eaten it nonetheless.
I don't see how all the groundwork in the world -- bear in mind I love NH-style groundwork and do loads of it -- is going to 100% prepare a youngster for everything life throws at him once he begins his career as a riding horse. Young horses are unpredictable. What are you going to do, never ride your horse until you are sure he will never ever spook no matter what happens? I suppose that solves the problem, since if you never get on your horse, there is no risk of you falling off. Then yay, you don't need a helmet.
If people don't want to wear helmets, fine by me, but if you're not wearing a helmet for totally daft reasons like, "My horse is so well trained I will NEVER fall off him," then you're a moron.