Sugar Free is Not unsweetened. It just doesn’t have ‘sugar’ in it- but rather a different form of sweetener, namely going by a different name. 

Sugar free is mainly for those with diabetes- whereas their body doesn’t process sugar like it should- hence a different form was produced so that diabetics can still enjoy that sweet sugary taste.

The only difference is that ‘imitation’ sugar is sweeter than regular sugar, hence making it more addicting- which in turn, in the long run can create weight gain (or dependency). 

The thing is… due to lack of knowledge. Many people eat ‘sugar free’ foods and deserts, unaware that it is better to eat regular sugar- except for the fact that marketing and is making people think they’re doing better with the alternative.