Can someone explain this to me?

That $500 is more than likely her deductible with her own car insurance company. Because the car was stolen AND uninsured, there is no one and no insurance company to take responsibility for the damage to her car. Your aunt more than likely has an "uninsured motorist" clause on her policy that covers her in the event that something happens to her vehicle at the hands of someone who is not insured. She certainly doesn't have to pay the $500, but that also means that no one, including her car insurance company, is going to help pay for the repairs or replacement.