Why is “May Contain” food labeling voluntary?

Why is “May Contain” food labeling voluntary?

In 2006, "top eight" allergen labeling became mandated by the United States Food and Drug Administration in all packaged foods manufactured and marketed in the US.  The top eight food allergens are milk, egg, fish, shellfish, tree nuts, wheat, peanuts and soybeans.  At that time it became easier for consumers to identify what packaged foods [...]