Brakes and Rotors

Maybe someone here can explain to me what happened to my car.
I have a 2001 E320 with 23,000 miles on it.
I took it in for the second service and my service tech called me and told me I needed new front brakes and rotors, which according to him were not a warrantee item.
I asked him how it was that my rotors were scored so badly without my brake pad sensors going off to indicate that I needed brake pads. He gave me a double talk answer and charged me $470.00 for the work. Later on talking to my sister, who has a 2001 Jeep Grand Cherokee (Chrysler-Dahmiler...hmmm) she said the same thing happened to her jeep at 20,000 miles and the dealership replaced the rotors for free. My dealership informed me that they no longer refinish rotors because of the thickness yada yada.
I dont understand how rotors can go bad on a car that still had viable brake pads unless they were defective rotors to begin with.
Please help me to get the information I need on this topic.
Thanks!