An article about veterinarians that pretty much sums up how I feel:

[Curmudgeoness]

11. I have to agree that the reason pets are obese is quantity of food.

When I had a cat who was gaining too much weight, I closely read the label on the food. I was shocked by the small amount of food that I was supposed to be putting out for him. Since that time, I have always used a measuring cup. And every food has different requirements, so if I change foods, I check for daily amounts.

I have no doubt that some vets are dupes, but I think that is rare. My vet that I use now does sell premium food, but the only time it has been suggested has been when a cat had a medical problem that could be treated with special diets. Usually, I have been told that any commercial food is adequate. No dupe there.