Could be that the Cycling club has insurance to protect those organising the group rides from litigation claims in the event of an accident. Then the insurers might be insisting that on such rides everyone wears helmets.I can understand the risks are slightly higher in a group - from the rest of the riders, not traffic - but I'd be happy to sign a disclaimer saying I'm riding at my own risk, but no.
That's why I don't cycle with them. Shame really, but hey - it may be their loss too, not just mine.