Right rear caliper failed under warranty. Why do I have to pay for the left side if it's mandatory they be replaced together?

As mentioned, the right rear caliper and rotor are being replaced under warranty (KIA 100,000 mile warranty). I understand that the pads are my responsibility, no problem there. I’m told and I do understand that the left rear caliper/rotor needs to be replaced at the same time. Am I responsible for the costs of the left rear rotor if they MUST be replaced together?