Let's get rid of all SAE standards, then we can be in the dark about everything. After all, manufacturers never lie.
J2807 sets minimum performance standards when towing and the tow weight allows for a 150 lb driver and passenger in the truck. Because it doesn't know what else may be loaded into the truck, what a pin or hitch weight might be, it says anything in excess of that must be subtracted from the tow weight.
J2807 is a performance standard, period and I think it is useful. The SAE is not psychic and only the owner can determine whether what he is loading and towing exceeds his trucks GVWR, axle and tire ratings.