The rear brakes do about half the braking of the fronts, give or take, but there still shouldn't be rust on the area of the rotor swept by the pads.

 

I've seen rocks or road debris get lodged in a pad and act like ceramic wearing away rotors leaving grooves. When they wear down or the foreign bit gets released, the pads then have a high spot and can leave rusty rings until the pads wears the rotor down.

If the rust is all over the swept area of the pads, you could have a collapsed line internally, or seized pot in your caliper.
Chances are you'd feel a little 'squirrel' action on hard braking though.