The history of this is relevant....I think..to those of us in the UK....
For 40 years or so, all Local Authority airports in the UK, which until the advent of Mrs T was virtually all public service airports bar BAA ones, charged airlines in accordance with a model agreed between the Airport Operators Committee (AOC) and the airlines. It was designed to, and did, encourage airlines to create new routes and airports to take a part in promoting and developing them. The amounts were decided by each airport, to preserve competition of sorts.
There was a fixed landing fee, calculated in accordance with weight. (The addition of Navigation charges, again by weight, occurred at some point as these things became more expensive and traffic developed needing Radar, Approach etc as well as the bloke in the tower with an Aldis and VHF set.)
And then there was a Passenger Load Supplement, based on the number of people getting off, or, sometimes, on.
The idea was that the Landing fee would just about cover the costs of providing the airfield, fire cover etc etc, while the PLS would add the profits if flights were full, after paying the marginal costs of handling the extra numbers. Thus, the airports became partners of the airlines in developing routes.
These charges were built into the advertised ticket price; there were no add-ons.
(Exeter was the first airport in the UK to offer (to JEA in the 1980's) a single charge based on the number of people, and no landing charge at all. It worked; that's why Flybe's in Exeter.)
And then in the early 1990's, I think it was, BA suddenly announced that they were being made to pay what it called "Taxes" by the airport, as well as the Government tax. With the greatest regret it had to add these to the ticket prices, and "Airport Taxes" were born.
The "Airport Tax" was of course simply the PLS that they had always paid and included in the ticket price.
Other airlines rapidly joined in, of course. Overall, the airlines managed to hike their revenues by 20% or so over what it would have been had they not done this. The travelling public and travel trade fell for it and blamed the airports.
And the airports did nothing about it, because they were afraid of the airlines.
The above is a simplified account! The sale of many airports was going on at around this time, which gave BA and the airlines the window of opportunity to do what they did.
The budget airlines used this system to develop their headline offers at ridiculous prices (1p, etc etc) and we now have the absurd situation of almost every expense of operating the aircraft being charged as an "Airport Tax", "Fuel Supplement", "Boarding Fee", "Credit Card Fee", Booking Fee", "Wheelchair Fee", "Baggage Fee", and so on, while the headline "fare" is a stupidly unrealistic and absolutely meaningless figure.
I'm waiting for "Piloting Charges", "Maintenance Charge", "MOL's Pension Fee", "Aircraft Lease Cost Fee" "Training Charges" sorry, scrub that, the staff pay those already, ......there must be others coming, too.
And Ryanair are still blaming their website for being "unable" to comply with legislation requiring the display of the full fare up-front.