Yesterday, a video of a man being dragged off an overbooked United Airlines flight went viral. The reason the airline claimed it could do so was that the crew sought volunteers to be compensated, but ultimately had their hands tied and needed to get someone off the flight.
But if you look at United's contract of carriage—the document passengers implicitly sign when flying with the company—things get fuzzy. As far as I can tell, there's one rule that speaks to this issue: Rule 25—"denied boarding compensation." It describes the cases in which someone can be denied boarding and what the airline can do if faced with such an issue. One clause says, "The request for volunteers and the selection of such person to be denied space will be in a manner determined solely by UA." Sounds ominous.
But there's the rub: This man was already boarded, so it's hard to see how the language of this rule applies. I reached out to United Airlines multiple times yesterday for clarification on this and have yet to hear back.