What is the difference between paying a landing charge for using an airport and paying a toll to use a toll road?
Lots! In the road case, one has a choice to take the toll road or not. In an airport there is no choice.
If there were boom gates leading into the towns that like to charge for use of their airport then maybe the playing field would be level...?? There is not, so I suggest that to charge for a facility such as an airport for casual use is nothing short of discrimination against those that choose to fly to that location!