I guess people let their guards down when the plant in the audience gets it right and win some money, they pay less attention to the scammer's sleight of hand.

Each round is double or nothing. The victim wins the first few rounds, but he starts losing after the scammer starts hiding the ball instead of putting it under a cup. The victim keeps doubling on, thinking they will eventually win, but it never happens.

In my case, they got spooked after my wife and I moved to the side, discreetly trying to see how the guy was hiding the ball. The false onlooker tried to block our view, and when we moved a little further to the right, he signaled the others and they flew. They moved so fast that if one of them had punched me for ruining their game, I wouldn't have known what hit me.

Maybe they were ready to go anyway, but since they work in tourist traps, they probably avoid violence to attract as little attention as possible.