Seems totally cut and dry. Jaguars are excluded per their terms which is a contractual agreement. Whether that is fair or not in your opinion does not matter.

Exotic automobiles include, but are not limited to the following models: Aston Martin, Bentley, Bricklin, Daimler, De Lorean, Excalibur, Ferrari, Jaguar, Jensen, Lamborghini,......

Obviously the terms are in Amex's favour, but that what you agreed to when you used the credit card. Caveat emptor.