This would just add to the overhead of running a restaurant. Also, thing of the liability induced if the calorie count is wrong. Spme trial lawyer will end up blaming some restauranter for their client's heart attack if they forgot to count the bacon grease they put on salads.
Nutrition information is already given a great deal of flexibility. I've heard of only one persecution, and that was a guy who purposefully sold full-fat danishes or ice cream or something as 100 calorie dieter treats when in actuality they were 900 calories.
So that 300 calorie candy bar may actually be 600 calories...