Almost all of that sugar will be naturally occurring in the yoghurt as lactose is sugar. If you look at a fat free natural yoghurt (where yoghurt is the only ingredient) it will still have nearly 12 grams per sugar for the same size portion. Then if you compare it to a fat free vanilla yoghurt that uses sugar instead of sweeteners, that will have about 21g of sugar for the same portion.
Whether I agree with them being free is a different story, but in this case - they're adding very minimal extra sugar. Sweeteners on the other hand, I would agree with a truck load!