An interesting look from a legal perspective at the now-infamous "re-accomodation" of a United passenger because United needed the seats for some crew going to the destination.
http://lawnewz.com/high-profile/united-cites-wrong-rule-for-illegally-de-boarding-passenger/
Based on this legal reading, once United had let folk board, all the "deny boarding" clauses no longer applied; while there are clauses regarding booting people off planes, doing so to let other folk get a seat is not among them.
This is one lawyer reading one set of contractual material — but it sure makes a certain amount of sense as provided. And given that David Dao is now taking the first steps toward filing suit against United[1] — well, I suspect we'll see how that reading applies.
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I'm sure all airlines have this. It's not limited to United.
+Stripes The Tiger True.