违法的是UA， United Broke Its Contract With Frequent Flyers

https://www.bloomberg.com/view/articles/2017-04-13/united-broke-its-contract-with-frequent-flyers

Most of the coverage of the United Airlines bumping debacle assumes something like, “United Airlines had a right to remove that flier. But should it have?” But a close reading of the fine print of the contract included in every ticket purchased from United Continental Holdings Inc. strongly suggests that United in fact breached its contract with passenger David Dao.

The contract allows the airline to deny boarding involuntarily in case of overbooking. But that’s not what happened; the airplane wasn’t oversold. And Dao wasn’t denied boarding. As far as we know, he was removed from a seat he had already taken after being assigned to it. The contract’s specific provisions for removing travelers or refusing to transport them don’t include the airline’s desire to free up seats, whether for its own employees, as in this case, or for other passengers.