Each ride has a couple different things. But from what I've heard from friends who have worked attractions, even the FP controls can range from preprogramed emergency announcements (Ride closed, evacuation of queue, fire) to E-Stops of ride in case of an emergency that occurs on their end.
While that may not sound serious or a safety hazard, someone simply hitting buttons and controls is a safety hazard to Disney and the CM was right to try and prevent it further. What if the pre-show door closed on someone's hand because the woman hit that switch? What if an emergency announcement prompting an evacuation had gone off because of it and some people stampeded out of the queue?
While that may not sound serious or a safety hazard, someone simply hitting buttons and controls is a safety hazard to Disney and the CM was right to try and prevent it further. If a child swinging on the chains in line is a concern, than some angry guest hitting controls is definitely one.