Brakes on ML 320

My dealer just told me I had to replace pads and rotors on my 2000 ML 320 with 30000 miles and nothing but normal suburban driving.

My sense is that if the rotors required replacement at this early stage MB didn't perform the right service at the scheduled intervals or the rotors were defective. We had previously changed the front pads at around 18.000 miles.

I understand the rotors cannot be ground.