Sorry for dredging up old posts, but physics does qualify as an "intrinsic reason" for this balancing method to work. And it does work quite well. Over the road truckers have been using it for years and seeing significant improvements in tire wear and life. I started using it on my last set of tires on my motorhome because the lead weights kept shifting on me (large diameter wheels are trouble that way).
When you start driving, the powder simply shifts about inside the tire until the shaking forces are minimized. The physics part of it is that it would require energy for the powder to do anything else but seek the the condition of lowest shaking force. My coach rides much smoother now than it ever did with lead weights and I get the added benefit of constant compensation for tire wear.
Alan King