Heh I know exactly what you mean. BMI is such a stupid system. Just go off bodyfat, it's a much more accurate measure of health. Average adult should be no more than 15%.
It's a little more involved than that, but that is generally correct. Your body needs a certain amount of essential amino acids to build muscle tissue. But protein alone is not healthy either. An average adult male that does not work out does not even need that much food proteins. A body builder however, needs a LOT more protein for muscle growth and repair. I would say a normal person needs no more than 20% protein of their total calories, while a bodybuilder would need upward of 40-50%.
It's also unhealthy to ingest little or no fat especially if you are bodybuilding. I would say about 20% of your total calories while body building should be fat. Thus a body builder would need about 40% protein, 40% carbs and 20% fat for a good balanced diet. Ideally the fats should come from nuts and poly and mono unsaturated fats. Your carb sources should ideally be low glycemic inducing high fiber foods like sweet potato, veggies and the like.
The average joe could do well with 20% protein, 60% carbs, 20% fat.