Original question was "why?". Well.....
At about the time that duty free was being canned within Europe some airlines became concerned that airports would put up their fees. They decided that they would split the fees out and show them separately in the tax box on the ticket. Agents weren't to chuffed about this because it removed an element of the "fare" on which they earned their income.
After some huffing and puffing the Association of British Travel Agents took a test case against some airlines (BA, LH and VS?) arguing that the airport fee wasn't a "tax". I won't go into all the ramifications but the agents won and the judge ordered that commission should be paid on the fee both from that point on and restrospectively.
At that point most airlines caved in and BSP (who collect the money from agents and pass it to airlines) went into a big calculation of who was owed what. The major hold out about paying was Ryanair and, I guess, this is the result.