It's a shakedown by the franchisee, pure and simple. I rented a Thrifty car in Cozumel for three days and paid the exorbitant fee for the 'total protection package' because I'd been warned up front. Checking in the car they actually got down and started running their hands along the bottom edges of the bumper flaring looking for tiny nicks in the fiberglass. Then they proceeded to tell me how great it was I bought the 'total protection package' because otherwise I'd have to pay $x for damage to the car.
Basically I paid my tourist extortion money up front.
I refuse to rent from Enterprise. Every time I've rented from them they come up with reasons to honor discounts made with the reservation and use aggressive bully tactics to try to get me to buy insurance I don't need.