Welcome to FT.
This really has nothing to do with Avis or professionalism. Presumably you knew the opening times of the agency and the timing of your flight at the time of booking, so you had plenty of time to make appropriate arrangements. Leaving a rental car in an airport parking lot with the key on the tire is, well, not very smart. If the car had been stolen (not unlikely in my opinion) you would have been on the hook, and the damage to your credit card would have been a lot greater. I doubt whatever theft insurance you took out would have covered such a scenario. I think that, under the circumstances, you got lucky here.