Asking people to ear less than they are eating now is thus asking them to be perpetually hungry. It can be done, even for a year or two if they have willpower, but eventually they will get sick of being cold, tired and irritated all the time --because that is what chronic hunger does to you-- and they will go back to the normal behavior of eating until they are no longer hungry.
Unless they are on a particularly aggressive program of calorie reduction, eating less than they are eating now is not asking them to be perpetually hungry.
Alcoholics are "making a choice" to drink alcohol. But we both know that it's more complicated than that. There are both habits and chemical addictions in play which makes it incredibly hard to stop.
Why don't you afford the obesity issue the same level of understanding?