Unfair charges - Very disappointed with service

We hired a car in Durban for 7 days and when we returned the car to the airport it was checked over and attendant said all was fine and in order. We proceeded to board our plane, as we were about to board we get a call to say that the passenger door lock had been tampered with and broken and that we would be liable for the repair. We said to the attendant that we had no idea about the broken lock or the car being tampered with as we hardly used it and kept it in the complex. He said Durban rental car break-ins are very common as ******* can see it's a rental car and they often break-in through passenger door lock. We were never warned about this when booking the car. If its such a common occurrence surely they would warn us and advise that the excess waiver option should be taken as in all likely hood the car will get broken into. They have charged us to repair the lock on the i10 car as well as their admin fee for damage which is excessive and have made a charge for refueling the car to take the car from airport to their own car depot?! How is that fair - we returned it to the same place we hired it and filled up the tank on drop off?!