How are the Fates not public domain?
Currently(or soon to be) Reading: Absolute Power, Batman/Superman: World's Finest, Birds of Prey, Green Arrow, Green Lantern, Justice Society of America, Shazam, Titans, & Wonder Woman.
First, The Fates are a huge part of a story where folks are saying "Boy, sure glad we got away from the Olympians!" As for Donna Troy, she is essentially a public domain character that DC let a writer slap a DC character's(who has nothing to do with the current character outside of a name) name on.