The vehicle classification system has not changed in decades while the vehicles within those classes have. Trucks have outgrown these classifications and limitations yet the manufacturers ratings are restricted by them regardless if the trucks can actually carry more weight. What we call a SRW 350/3500 used to be a class 2b vehicles with a GVWR of under 10k. It would be suicide for vehicle manufacturers to stop making trucks with a GVWR of under 10k due to all of the old laws and regulations that go in affect once you go over 10k.
I think people mistakenly believe that the number in side of the door will tell you what class it is in. This is false. Today, 150/1500 trucks are class 2a, not class 1. Class 250/2500 trucks are in 2b and in class 3 now. You can have a 350/3500 truck de-rated to a class 2b with a GVWR of 10k. A F450 is in class 3, and even some F750's are in class 8. The numbers are just a name of the model and the limitations of vehicles in certain classes has more to do with outdated laws than actual safe carrying ability.
And no, the manufacturers GVWR rating is NOT enforced by the law. Only the registered GVWR and road axle limits are enforced by law.