According to CBS NEWS, Under the U.S. Food Labeling and Consumer Protection Act, food companies must identify major allergens if that food is an intended ingredient. These foods include wheat, egg, milk, peanuts, fish, shellfish, soy and tree nuts.
CBS states, “However, if food is produced on shared equipment, there’s still a risk that some trace of an allergen may be in the product. As a result, food makers began adding what is known as precautionary allergen labels. They state that a product “may contain” a specific allergen or that a food is “manufactured in a facility” that has products containing a specific allergen.
Those precautionary labels are voluntary, both in the United States and Canada…”
In a survey, however, about half of the respondents thought those precautionary labels were required by law. See: http://www.cbsnews.com/news/the-confusing-state-of-food-allergy-labels/