Well I'm sure FNG would be better able to comment on this, but I imagine that if your personal risk assessment concluded that the risk was significant enough to warrent the wearing of head protection, then, as you owe your passengers a duty of care, you would be required to give them head protection too. To do otherwise would apprear negligent.
If you didn't and they suffered a head injury that the protection could have provented, or reduced it's severity, then I think you'd have a hard case justifying why you saw the need to protect yourself, but not your passengers.
dp