some of you keep bringing up "copyright." The hamburger is in no way protected by copyright. Don’t blame copyright laws for people using false advertising of a product that cannot be protected by copyright in the first place.
Hardees does not own a copyright for the Big Shef. No one can own a copyright for a hamburger. If you see a copyright notice on the advertisement it is either for the photograph or for the text on the poster, provided the advertisement contains enough copyrightable text. Protection for the protograph does not protect the work contained in the photograph, just that particular photograph.
So, Hardees is not bringing the hamburger back to satisfy any copyright issues, because they don’t own a copyright in the hamburger.