B.S.In what jurisdiction is 'kidnapping' by larceny legal?
No one is going to enforce such a contract at the point of a gun, and doing so would be illegal.
Freedom cuts both ways. You have a right to enter into a contract -- or not. You have a right to limit your options in exchange for a lower price. If you then choose to ignore those self-imposed limitations, the other party has the right to collect the higher price you would have paid had you not intended to cheat them. A court of law is where such rights are protected (with force, if necessary), and adjudication in a court of law is available to all.
It is patently obvious they have no right to use force to prevent you from leaving the plane (at a time when other passengers are also allowed to leave the plane). But I don't believe anyone argued that they did.