I'd figure the right thing to do, on the airline's behalf, would have been to rebook the passengers on the next available flight or rebook them on another carrier. After the FBI cleared the passengers, technically the airline had no right to refuse carriage, unless there was a credible security threat. Either case that would probable fall back to the FBI or TSA.

I'm not an attorney, but I'm thinking that the airline could find itself in a civil suit for refusing to carry these passengers after they had been cleared by the FBI. Again just an opinion, and that assumes that the airline offered them a refund only, and told them that they were not welcome on the flight.