Not sure why you're expecting a legal reference to common sense.
On a large jet, the pax weights will average out so it doesn't matter if you've got a few obese people onboard. Jets are also somewhat less performance constrained.
On a light aircraft, W&B and performance limitations are somewhat tighter and so if you're planning to operate close to the envelope then it's in your best interests to use actual weights. As numerous avoidable accidents have demonstrated otherwise.
Previous discussions on the matter on PPRuNe have pointed to AC119-4 as being one of the more detailed documents on the matter.