Nasty Boys

Recently I drove to the supermarket just down the road (on my own for a change). I was stopped at the traffic light, and there were a group of 7 policemen in varying locations at the crosswalk. I figured they must be looking for someone.
As I watched (and the lights at the intersection in question are indeed very long), the police systematically stopped all the men on motorcycles, and ordered them to place their hands on their helmets. There was an entire queue of motorcycles between cars poised just so delicately balancing their bikes while their hands were in the air.
The police took each man in turn and grabbed them by the thumb first and then twisted it over the palm of their hand. They did so with extreme agression and held tightly to the motorcyclists left hand while patting them down in what appeared to be a search for weapons.
The procedure was forceful, intrusive, and almost violent in nature. The men succumbed in a willing and docile manner. It was surreal to watch.
What I still can't figure out is what happens to the right hand of the motorcyclist? Aren't most people right-handed? If they wanted to attack, wouldn't they immediately and instinctively use the right hand?
Anyway, when I spoke to the security gaurds at the supermarket & pointed out this unusual activity asking if there was a recent crime they were screening for, they shrugged and said, "Necessary precaution. Lots of crime in traffic and the majority carried out by men on motorbikes." "It's normal," they said.
I think living here in Sao Paulo my whole sense of "normal" has gone totally down the toilet. And I thought Thailand was bad.