Why did front rotors and brake pads have to be replaced after one year?

KIA replaced my front rotors and brake pads Aug 2014. One year later my brakes were not functioning well and KIA said the right front rotor was grooved because of rust and should be replaced, adding I should do both to keep them even and change pads, which were hardly worn. Are they ripping me off? My car sits outside in a plastic temple garage. I live in 4 seasons - winter lots of snow, summer can be very humid for a couple of weeks. I do not drive my car often as I'm retired. Can non-use speed up rusting of rotors?