This is nomal. I have this on mines as well but not everywhere. It's due to the climate that we live in (generally the winter). That area is bare metal and does not have a protective finish on it to prevent it from rusting.
It eventually happens as the rotors/hub are exposed to the harsh climates that we have (plus the added salt in winters.)
I suggest maybe using a wire brush to clean it up a bit, then use some sand paper. Unless you get it professionally painted, the rust will come back.
I just got my rear rotors changed and I can slowly see the rust forming (not bad as it is still summer). I wouldn't be surprised to see a nice thin coat of rust once winter is finished.