User blog comment:Woohoot/Things that totally could happen, but won't cause... Manga.../@comment-10543621-20150524125222/@comment-5398287-20150526125719

I think the salt weakness for the Kage Kage no Mi is more or less just a reference to salt's "holy" or "anti-evil" properties in a lot of fiction dealing with the underworld and the like. Not necessarily a weakness for other Devil Fruits.