In my experience, what the officers told you is correct. That is, you tell the DMV the weight to register for the maximum gross weight. Then they charge you fees accordingly and issue the registration.
I believe that their thinking is that, because of the extremely large number of configurations across all models, the DMV relies on the owner's knowledge and representation of the required registered weight. On a DOT inspection, the officers will check the registered weight against the scale weight. If you're overweight (either by total against the registration or by axle regardless of the registration), you can expect a citation.
A registered weight over 26000 pounds or if the vehicle has air brakes requires a CDL. That's why you see rental van trucks like Penske or UHaul have a maximum weight of 26000 painted on the doors.
If you're involved in a crash, all bets are off. As a previous post indicated, crash investigators may scale the truck. If structural weights (tires, door label, etc) are exceeded regardless of the registered weight you may be subject to a criminal citation as well as be civilly liable.
Lastly, if you're on antique tags, have private tags, or insured as an antique or as a private vehicle, generally speaking, you may not engage in commercial operations such as hauling, towing, and so forth for someone else or for your own business. If you do, you may be cited, and in the case of the insurance company, you may not covered.
Hope that helps.