I think the idea of an "unlimited" diet is one where every type of food is at a person's disposal. Fruits, veggies, and red meat are all there in the same quantities as candy bars and cake. Given that kind of availability, the theory is that people would generally take the food that's overall more healthy (that's not to say they'll never take the junk food). If I understand it right, this stems from the idea that people want what they can't have. If something is readily available, then they have less of a desire for it than if it was restricted from them.