Buying local produced products and taking it back to the ship is normal behavior for sailors from all nations.
That is true, as I have purchased things while on ACDUTRA to bring home. What is lacking is how often do warships pull into an Australian port so that their sailors can purchase baby food. If this happens frequently, then I guess it is a combination of personal use and black market income. If it’s not very often it points to something maybe more ominous.