Why do products which contain aspartame have the label 'Contains a source of phenylalanine' on the packaging?

This label is there to help people with the rare inherited genetic disorder phenylketonuria (PKU). People who suffer from PKU cannot metabolize phenylalanine from any source properly and need to follow a very strict diet. The disorder affects approximately 1 in 15,000 babies born in the US. All babies are screened shortly after birth for PKU.