I think Burgess gave you the best answer to your question,
Every element of any program should be kept in context. In my understanding, these are the principles of the McDougall Program regular diet:
1. Whole foods.
2. Plant foods.
3. Starch centered.
4. A wide variety in each of three cateogories (fruit, vegs, starch).
5. Eat as much as you want until full.
The fourth element kills the idea of eating nothing but oranges or nothing but corn. The fifth element kills the idea of compulsive eating.
It is important to focus on all the fundamental principles not just one.
Unlimited does not mean unlimited in the sense that you can eat all you want of anything.
What it means is that if you follow the principles of the program, especially of the Maximum Weight Loss program, you will be able to eat all you want until you are comfortably full, and still lose weight.
The reason, as TominTN pointed out is due to calorie density. Many many studies have been done in the last few decades confirming this. If you allow people to eat "ad libitum" or all they want till the are comfortably full, from low calorie dense foods, they will lose weight, not be hungry and do not have to count calories.
Of course, calories still count, but it becomes almost impossible to over consume calories from the foods you choose if you follow these recommendations.
The numbers Tom gave are very close, so let me adjust them slightly
These are averages
Fresh Veggies are around 100 cal/lb
Fresh Fruits around 250-300 cal/lb
Starchy Veggies/Intact Whole Grains around 450-500 cal/lb
Legumes around 550-600 cal/lb
Processed Grains (even if their Whole grain) around 1200-1500 cal/lb
Nuts/Seeds around 2800 cal/lb
Oils around 4000 cal/lb
What they found is if the calorie density of the food is below 400 calories per pound, not matter how much they eat, they all lost weight.
Between 400-800 calories per pound, with some moderate exercise, they all lost weight.
Between 800-1200 calories per pound, people gained weight, except for those with very high activity levels
Over 1200 calories per pound, everyone gained weight.
Remember, the physical sensation of "fullness" is influenced in a large part by the filling of the stomach and the triggering of the stretch receptors. This would happen regardless of the calorie density of the food, as long as enough food was consumed.
However, between 400-800 calories per pound is the range where people either maintained, gained or lost a little. It was the area that I call the "cut-off" zone and the results depending on the person and their activity level.
I would not worry to much about the exact numbers when you are trying to apply this to yourself but would be more concerned about the principles as I would not want anyone to weigh and measure their food. The calorie density numbers I give for food groups are "averages" and dont apply exactly to each food in the group.
If you follow the MWL program, you will be applying the principles of calorie density. If it is not working as well as you would like then you can adjust the calorie density of your intake by making slight adjustments in your food choices.
These numbers are also inline with other recommendations.
The recent WCF/AICR report on cancer recommends that the average calorie density of our diets be around 550-600 calories per pound, to avoid obesity and weight problems.
The Okinawan diet, before Western influence, was around 600-650 calories per pound
So, knowing all this, if you look at the numbers, it all makes sense.
A starch based diet, made up of starchy vegetables and intact whole grains along with some fruit and veggies, will have a calorie density under 500 calories per pound and maybe even 400 calorie per pound. It would be near impossible to overeat.
You can also see the problem with many of the "low fat" diets that focused on processed whole grains, like whole wheat bread, crackers, dry cereals. At 1200-1500 calories per pound, if they become a large part of the diet, they can raise the overall calorie density and make it much easier to overeat on calories and easy to gain weight and/or not lose weight, even with a higher activity level. Hence the principles of the MWL program is to avoid those foods, or really limit them.