then you could legally fly any plane (even the big iron) as not having a license does not make you part of the flight crew?
An aircraft needs to be flown by a legal crew. So you can't fly an aircraft without a licence (except on a solo endorsement) on your own.
But if the legal flight crew requirements have been satisfied, then it's perfectly legal for you, as a passenger, to manipulate the controls. It doesn't absolve the flight crew from staying legally responsible so it would be, well, a little careless of them to leave the flight deck and go take a nap for instance.
Have to say that letting passengers having a go at the controls is something that is typically forbidden by SOPs of large airlines though. And by law passengers are not allowed to enter the flight deck of a commercial airliner in the first place.
A friend of mine had a passenger doing rolls in his RV recently on her first flight
I prefer to let my passengers do a loop rather than a roll. There's less chance of things going wrong there, it's an easier skill to master (pitch only, not a complicated pitch/roll combination), and it's less nausea-inducing. So you can let them try it a few times until they get it right.