Actually, State Farm only has to pay out according to the terms of the policy, and most policies have terms that can allow the insurer to limit payments below full restitution. An insurance policy is a contract between the insured and the insurer, not between the insurer and a third party. For instance, if a car owner only buys $50,000 of liability insurance and is at fault in a wreck that totals another owner's $100,000 car, the insurer only has to pay $50,000. The insurer's obligation runs to indemnifying the person who bought the policy up to the limits of the policy and to no one else. If the owner of the $100,000 car wants to recover his full damages, he has to collect the difference from the person who was at fault in the accident (or, more typically, by filing a collision claim under his own policy).
In the case of my accident, in which both I and the at-fault driver were insured by State Farm, State Farm refused to reimburse me, even on a pro-rated basis, for the loss of my $10,000 radar/laser detector installation and my $3,000 Opti-Coat ceramic coating, as the policies contained a clause excluding coverage of aftermarket equipment or applications. I would have had a strong case to go after the other driver personally, but the time and litigation costs -- plus the fact that he was a young kid who was not driving aggressively and who was very contrite and completely honest with the police -- induced me to let it slide.