I'm not a lawyer, but I have to wonder at what point this becomes something I can best phrase as "fraud through gross negligence" (i.e. there wasn't an express intention to deceive, but the company simply refused to change their advertising over a period of time in spite of the offer being withdrawn). In this case, there's a website bug resulting in the wrong fares being offered. They know about it. They don't fix it. They're repeatedly informed of it, and repeatedly decline to fix it. At some point that does cross a line from either an incidental bug (which happens) or some mild neglect (which also happens) to the decision not to fix the bug being a conscious decision.

The best analogy would be a store which has a sign in the aisle saying that a box of cookies are $5 plus tax...but when you go to the register they ring up as $6 plus tax. Ok, maybe the $5 was a sale and the sign hasn't been taken down yet...but if you come in day after day and week after week and they still have the sign up but the price is still ringing up wrong then eventually you cannot help but conclude that the store is intentionally deceiving you.

Edit: I'd also note that this would be a particular problem if, for example, Delta was showing $300 (when the price is actually $347) and United was charging, say, $345 for the same flight.