Junk food would be soft drinks like Coca Cola, Dr Pepper, sugary foods like twinkies, no nutritional value foods like potato chips.
Those are classic examples of junk food. It's not like they are a potato that can be nutrition-downed by the consumer. It's food that's had that done before it gets to the shelf so that it has no merits to it's existence except to destroy the health of the person buying it. White bread would be a classic example IF...
Never fear though, if laws were created penalizing it, the manufactures would simply do what they do to white bread. Add a bunch of enrichment, like vitamin powders and what not. Then it wouldn't be junk food any more.
Again, it's cherry picking of laws. In some areas, parents can be jailed if their kids do not attend these gang infested, violence riddled, low competency, broken down schools on a regular basis yet they are free to feed them a diet of snack foods like Doritos, high fructose corn syrup infused Dr Peppers and plenty of sugary desserts at will.