Agreed, totally against the laws of physics and completely unbalances the bike
The "grab handles" on the back of the bike are for manoeuvring a bike into and out of a parking position and are absolutely not meant for use in transit.... Its certainly something you will never see a Thai doing.
From what I have observed (I have spent a lot of time in Pattaya's ERs!!) Most of the farang that you see with their an arm in a sling managed to break their collarbone or dislocate their shoulders by using said handles