Too easy to work around and too hard to define. Look at a lot of "No added sugar" sweet things - they can use fruit juice concentrates, or plant syrups etc. Then there are artificial sweeteners. Then there things bred for sweetness.
I think it's the hard work arounds that will eventually break it. Like we're already growing strawberries that are naturally sweet, in the future maybe our bread will be made of wheat that has "natural" sugars in it or something. But I feel that for that to happen the "no added sugars" labeling would have to be exceptionally effective.
What if you add it to a soft drink? Or a ready meal? What about using it instead of fruit in a ready meal?