Once an airport management decides to take on fees related to safety then they are responsible for an extra level of safety. When they fail to deliver, they will be sued and the insurance companies will step in.
Unfortunately, we have to have an ‘ADS-b fee’ accident before this money grab ends. For example, what if a pilot has not paid his extortion and the airport refuses permission to land? What FAR is the airport management going to use for landing denial justification?