Passengers manipulating controls

Is the "no touching of controls" when it comes to insurance common in the power community? If that was part of the policy in my previous glider club, I didn't know it. It certainly wasn't in the club rules. I routinely let passengers try a couple of turns at altitude if they wanted to. Usually, they didn't but sometimes (especially young teens) wanted to give it a try. As far as I can tell, this is not prohibited by by regulations. Am I missing something here or is this just the insurance people being overly cautious? It seems a bit ridiculous.