SW doesn't work as a calorie control diet but all foods obviously contain calories. The premise of SW is that you eat superfree, free, syn'd food (in that order if possible) when you are hungry.
If you eat them 'just because they're free' you'll be consuming more calories than your body requires for normal metabalism - and therefore it can slow down your losses.
That said, if you get an attack of the munchies, it's much better to have a yoghurt than to reach for the biscuit/chocolate tin or crisps.