I can understand your annoyance.

The airline should have been aware of it - all rental car companies charge an additional airport fee all over the world and have been doing so for a long time - at least 10 years or more.

In many countries, including Australia, this would be a breach of fair trading laws and leave both the airline and the car rental company open to fines and/or prosecution. This happened to Avis in Australia a few years ago.