As long as they are wearing evenly across the width of the tire and your tire pressures are within an acceptable range, the wear is normal. It is not unusual since it is a rear wheel drive vehicle. The rear wheels don't have to spin to wear quickly. Just accelerating briskly with no apparent wheel slippage will significantly reduce the life of the rear tires. I have 20K miles on original Michelin PS2 tires. My Crossfire has an autostick and unless you're accelerating from a stop while turning, the tires won't typically spin. My rear tires have less tread left than the fronts. I expect to get as much as 30K miles with mildly spirited driving. I may even replace the tires in 1-2 years based on age before they completely wear out (6+ year old tires with lots of cosmetic surface cracking developing).