If the insurance company can't link passengers touching of controls, in general, to accidents why forbid it? I wonder how many GA accidents have been linked to a non-pilot passenger handling the controls. There are probably some but I'd guess that they would involve panicked passengers. If its such a big deal why not require that there be no controls accessible to the passenger? I could understand no touching during critical phases of flight (approach, landing take-off, below x altitude, etc) but to completely forbid it is overkill.

I just fail to see the logic of "Well, before you stall/spun on final you let the passenger make a few turns an hour prior. Therefore, your policy is cancelled" or worse, deny coverage on that particular accident.

I know 'don't ask, don't tell' is the simplest way around it but I'd hate to say to someone "I'll let you try but you can't tell anybody when we get back on the ground".