Here is a Wikipedia answer for you:
Note that GVWR is ONLY for a vehicle WITHOUT trailer. And, yes, this is true in all state and federal statutes. Like I said, there are more calculations that are required for trailers but the average person is unwilling and/or unable to do them. It comes down to axle spacing (moment appiled about the loaded axles) and weight per square inch of tire contact. That is why I suggested you do a free body diagram. The actual load becomes a function of where the load is in the trailer. This is why most horse trailers have the horses just in front of or on top of the axle. There is no moment applied to the rear axle of the tow vehicle.
And this is why there is pretty extensive software to make the calculations. The over simplification is the calculation I presented which anybody can do and overestimates the load limit, thus applying a factor of safety.
With a trailer, the vehicle ceases to be a two axle truck. It becomes a 4 or more axle vehicle (for most horse trailers). Thus the loading per axle becomes a function of moments and direct loads. Hence why my 4,500# trailer only adds 750-1,000# when hooked, and not even coming close to the GVWR.
The physics doesn't lie.
Take the extreme example of a semi that hauls 100,000#. Do you think the tractor must have a GVWR of that?? No truck on the highway does.