The whole purpose for me placing my RH thumb on the RH side of the rifle is for rapid bolt manipulation of a bolt gun, if I even do it. Depends on the stock/frame design of the bolt gun. It just allows my thumb to be right there as soon as I my trigger finger comes off the trigger and lifts the bolt knob. The thumb prevents me from over-rotation and unnecessary movements for mechanical efficiency of rapid-manipulating the bolt, then having a digit there to return stroke the bolt and lock the action down again.
When I see guys doing it on ARs, I think to myself, “Do they know where that came from and why?"
You see a lot of the YouTube generation doing this with ARs, trying to look cool, but it doesn’t make sense. Not hating, just pointing out where I think it came from and why it doesn’t apply to the AR-15/AR-10.
A lot of these guys belong behind the camera and editing, not in front of it. In fact, most of them don’t belong in front of the camera yet, but will do really well some day.