Sainsbury's own brand kids' fromage frais used to have no added sugar, and just use fruit puree, but I recently noticed that it does have added sugar now. So I emailed customer services, and got a very helpful reply, which I think demonstrates that it's not just a case of fruit = good and sugar = bad:
"Thanks for your email. I?m sorry the fromage frais yogurt we?re now selling in store appears to have more sugar in them. I can understand your disappointment, especially as you want to feed your children healthy food.
All our products come from reputable suppliers. We visit them on a regular basis to ensure they meet our high standards. I?m therefore disappointed we?ve let you down on this occasion.
We?ve contacted our technologists who explained that our previous recipe used de-ionised fruit juice to sweeten the product.
This is a very concentrated fruit juice in a syrup form which is effectively sugar and is considered to be no more beneficial than sucrose from a health point of view.
Due to de-ionised fruit juice not being as sweet as sucrose, this means that more of this sugar is required for the same level of sweetness.
We?ve decided that this could be misleading to customers and therefore decided to adapt the recipe using a mix of the juice and sugar to give the best flavour, rather than make it appear as if products contain no sugar when they do in a less obvious form.
An added benefit is that as less has to be added, the total sugar content (declared on the packaging) has in fact been reduced from the previous recipe.
We can confirm that even if no sugar was added to our fromage frais, it would be illegal to make the claim ?no added sugar? on the pack. This is because de-ionised fruit juice is deemed a sugar.
Although some major brands make declarations on products such as ?naturally sweetened with fruit juice? or ?fruit juice concentrates?, this is effectively highly concentrated juice.
I hope this information is helpful to you and your children continue to enjoy our buying and eating fromage frais."
I have to say I was really quite impressed about the detail in their reply (and they sent it pretty quickly after I contacted them). And although I am generally quite cynical (ie they are obviously trying to keep me buying Sainsbury's kids fromage frais), I think this sounds like a plausible explanation.