I've wondered the same thing before. Is it for commercial vehicles only, DOT stickered trucks only, over 26k lbs only, or what? What if you are driving a 3/4 ton truck pulling a tandem dual 20k gooseneck? Would it make a difference if you are hauling your own equipment and running a "FARM" tag?
And if you do have to stop, in what ways can they determine that you are overweight? Do they go strictly by the # of axles, the GCWR or the truck, the GVWR of the trailer, or all? I guess the weight ratings are specified by manufactures, but does the DOT use these to see if a truck is overweight?