They realize that if they tap car rentals to tax instead of something else, the voting public probably will not be angry with them. Most of the time, when they raise taxes on rental cars, they are taxing people flyng in from somewhere else. Those aren't voters who can affect local elections. There's no problem hitting them up for a lot of extra fees and taxes.
If you do want to pay those extra greedy taxes and fees, there's a way. Don't hire from the airport. Those are the rental cars the politicians want to hit up. Instead, hire from downtown. Those are likely to be cheaper.