Labelling them is arguably a "warning".
And arguably, it's truth in advertising.
There are labels of all sorts on food. There's a little "K" in a circle that means kosher and there's a symbol for Halal and there's labels that say "Grown in California" and "fresh" and "delicious". Those are not warnings.
There is nutritional information, there's a list of ingredients. If consumers want it, why not a simple little symbol that shows the food was grown from GM organisms?
Remember, it's the consumers that are paying for all the GMO research, for all the products, for all the salaries of all the scientists, for all the marketing and for the lobbyists trying to get the federal government to pass industry-friendly laws.
When an industry tries to get laws passed which are meant to make sure customers DON'T get what they want, it raises red flags.