The reason is this.....for example....oranges are free, right? But if you juice them, they become synned This is because you would need at least 8 oranges to make a small glass of juice....but would you eat 8 small oranges? Probably not!
And then, when you've drunk your glass of orange, you'll probably want something to eat too.....whereas if you'd eaten 1 or 2 oranges, you probably wouldn't.
Food optimising is all about satiety..SW say foods are free, but they are assuming because they are filling foods we won't over indulge. not always the case, I know!
hope that explains a bit for you?