There are two very different scenarios at play here:

(1) Airline posts incorrect fare for whatever reason. You attempt to book it, they catch it before you book it, and you complain about not being about to book the incorrectly-low fare. In this case, while I understand the disappointment, I also understand the airline's right to not sell that fare. I liken that to a typo in the Sunday paper offering TVs for $29.99 instead of $299.99. Joe's Electronics can't be expected to go out of business because the Tribune made a typo, can it? (Of course, I tend to agree with cordelli on *some* of these low fares. They may not be unintentional, but that's another story.)

(2) Airline posts a fare (whatever it is), and then ALLOWS YOU TO BOOK IT. That's the key difference. If the newspaper prints an ad for $29 TVs, I drive to Joe's Electronics, Joe sells me one for $29 and helps me load it into my car, and I get home to find a message from Joe saying "Oops...it was really $299 - please come back and pay the difference", then it's his fault. I wasn't unethical buying the TV (even though it was obviously below cost), and Joe has no right to take it back from me.

THIS is the scenario from United's $50 Europe fares and Delta's RPU Gestapo. IMHO, the airlines have NO grounds to stand on. Am I unethical to select a ten-segment mileage run for a low price if the airline is PERFECTLY WILLING AND EAGER to sell it to me? No - no question about it. Am I unethical for selecting the lowest fare to Europe, even if it seems to be lower than usual? Again, the airline is PERFECTLY WILLING AND EAGER to sell it to me. No - no question about it. Is it unethical for the airlines to change the rules and try to jam a customer who has already bought the ticket? YES - no question about it.

In general, I'd like to consider the airlines as a FRIEND. Why? Because I kind of like having them around, and I certainly don't want to go to Europe or Hawaii in a boat. Some airlines are certainly less friendly than others, as we all know...