More just a result the rotors are cast iron. Any unpainted surfaces that are not swept by the pads will rust. Exposure to rain or just wash/rinse water will have the naked iron surfaces with a coating of rust. If one drives the car in the winter slush/ice that gathers under the wheel wells rains salty water down on the brake hardware as the car sits and this stuff melts.
The rust is not a problem -- though I caution one to use the brakes hard enough to remove the surface rust from the rotors after taking the car out before he needs to use the brakes to bring the car to a stop to avoid possibly ending up with pulsing brakes -- unless as someone I believe has already noted the vented portion of the brake rotor is full of rust.
I recall when I picked up my "new" (used) 996 Turbo the front brake rotor holes were almost all blocked with rust/brake dust. I had to take a rough round file and clean each hole out. I checked and the vented portion was free of any significant rust build up. Sure the inner surfaces were rusty but air flow was not impeded.
There can be some rust develop between the rotor and the hub face. So when the rotor is removed to be replaced it can be a bit difficult to remove the rotor from the hub. (Be sure to remove the two counter sunk machine screws first!) A tap from a lead/soft hammer can help. Then afterwards the hub face to which the new rotor fits against needs to be cleaned of any rust build up. I don't recall if the factory calls for anything like grease to be applied to this surface. Probably not. Just clean and flat is all I recall it wants to be.