Eastmids
I think perhaps your suggestion that those who complain are "stupid" is a tad harsh, however the British, as I have oft commented in the past, "know the price of everything and the value of nothing", and as you say, there is "no such thing as a free lunch"
I doubt your average traveler understands how an airport works financially, and certainly won't realise that every cup of coffee, car parking space, bottle of pop from the duty free etc has a rake off taken by the airport. Many of them, when they go on their annual holiday probably didn't even know which airline they are travelling with until the tickets arrive!
They are captive audience, ripe for the taking! That (as a cockney) rep from Balkan Holidays once said to me many years ago on a holiday in Bulgaria is "free enterprise mate"!