Well, I have to disagree.
Prescribing drugs without a license is illegal, ok, but if you are a doctor, it is also illegal to prescribe drugs for sports performance or for the look.
How would someone prescribe a contest diet being a nutritionist? Or an exercise routine being an exercise expert? A contest diet is not a diet to improve health or well-being, dietitians are not given education / knowledge to get ripped with their clients.
And if they apply that type of diets with lean clients they would be violating the Hippocratic oath.
Is not the same a hypocaloric diet for fat man with 35% fat to take him to 15% fat than a hypocaloric diet to take a healthy and fit person from 12% fat to 5% fat, in addition to the diuresis process.
And well, if the law had to punish every person in the world who prescribes drugs, diet and exercise without a state-regulated license, there would be no prisons to lock everyone up.