Holden Barina 2001: Brake pads and discs need replacing?

I own a 2001 XC Holden Barina that has done 77,000km, is still under warranty, and has been serviced by Holden dealers. At the 75,000km service it was noted the front disc pads needed replacing, and when the job was completed the dealer noted the disc rotors were at the minimum thickness and should be replaced. Is it acceptable that every time the disc pads need replacing, the rotors should be replaced as well? Given that most of my driving is on highways that require minimal braking, should the disc rotors wear out so quickly? Apparently, Holden considers rotors a replaceable item and the disc life is considered quite acceptable, but as they are $398 a pair, and genuine pads $140 plus labour, I think upwards of $600 per pad change rather outrageous.