The rust/corrosion that you see on the rotor is because the pad is not making contact with the rotor in that particular area. Your whole rotor would look this way if the pad didn't scrape them clean when you stop. The rotors do not always wear completely evenly and when you throw new flat pads on used/worn rotors, there are some places where the pad will have to wear into the shape of the rotor. This has nothing to do with the the pad being too hard on the rotors. After some use, the pad should wear in and scrub the entire surface of the rotor.