It's basically a 2 fold reason. Firstly if you eat say a cous cous salad you would have X amount and it would be fill you up. If you have the same amount of couscous, but in a cake, it probably wouldn't fill you up.
Secondly it's also retraining your brain. Cake = healthy isn't true. So just because you make it differently doesn't make it any less a cake, sure it might be less in calories and syns, but still. It's the same with roasting chick peas to make "peanuts" and the lasagne/potato strips in the microwave to make crisps argument.
Cake, peanuts, crisps, these are all foods that probably helped you gain weight, so by making them "free" if you make them differently is telling your brain "cakes, crisps and peanuts will help you lose weight", which isn't true. Training your brain into saying you can X amount of these foods to keep losing weight is fine, and a better snack food is fresh fruit or whatever.
I feel I may not of explained that second point very well, so please ask questions if it's confusing!
Oh and with regards to this argument; "couscous/rice is free, so how come if you grind it up it isn't free?" It's simply because a.) your body doesn't use any effort breaking it down and b.) it isn't filling.
Free foods are free, because they are filling and have a low calorie density (the minimum amount of calories for their volume).
Hope this helps,
Natt xxx