It's more than just being detained. It's the damage to one's reputation. Everyone at that gate and on that flight thinks the guy might be a terrorist. That is significant harm to one's character and reputation. $400K is a fair price to pay for ignoring the fact he was cleared of any suspicion by the police. He was simply in the wrong place at the wrong time. Plus, in most our of court settlements, the airline would not have to admit to wrongdoing. When ruled against by the courts, the airline was proven to have committed wrongdoing against this passenger.
Unless he was traveling with fellow employees or customers, I don't think his reputation was done much harm. He'll likely never see his fellow passengers again. On the flip side TSA is an unnecessary PC joke. There will never be another successful hijacking of an American airline again.