Rear Brake Pad Problems on 2000 ML430

My 2000 ML430 has 35,000 miles on it and it has now worn through its 3rd set of rear brake pads. It had the fronts replaced at 30,000 miles. The car stops fine and doesn't feel strange, however I think it is unusual that the rears are wearing out 3x faster than the fronts. I took it into the dealer for warranty (where it is right now) and they said it was normal and that if I don't pay them the $250 to replace them, they will charge me a $50 inspection fee...Help.