Since there is no evidence that GMO's are harmful, labeling should be the other way around.
It's more logical for a company that wants to appeal to consumers like 20years_to_get_free to label their products non-GMO than to force all food that has GMO's to have the label.
You don't see Celiac's folks demanding that foods be labeled to indicate they contain Gluten. No, they go to the store and buy foods that they either already know are gluten free, or they are labeled Gluten Free.
And there is hard evidence that people that suffer from gluten sensitivity and Celiac's disease are adversely effected by Gluten.
No such similar evidence exists on the adverse effects of GMOs.
That's why it's far more reasonable for foods that are nonGMO to label themselves as such.