It's probably required by law.
Hear ye, hear ye! All citizens let it be known! Henceforth and heretofore, it shall be decreed and shall be known by these presents, that any product containing peanuts shall herewith be required to carry a written warning, to wit, to state clearly that it doth, in fact, indeed and of a surety, CONTAINETH PEANUTS, in order that anyone puchasing or contemplating the purchase of, said product shall know that it doth indeed contain the nefarious and potentially deadly allergen commonly known as and referred to as:   PEANUTS.