For all the people who keep insisting that inside the plane makes a difference:
After he was removed the first time he broke free and ran back into the airplane. Are you defending that, also?
It needs no defence. If he was entitled to keep his seat (and he was), then he was entitled to resist attempts of any kind to forcibly remove him from it or to restrain him from returning to it.
Authorities must be obeyed if, and only if, they are acting lawfully. Resisting unlawful authority is not only acceptable; it is a moral duty which we should applaud anybody for refusing to shirk.
Your appeal to the sensibility of craven cowardice in the face of abusive force is misguided and pathetic. A uniform does not confer upon its wearer the right to initiate violence on a whim.
That's not how US law generally see it. So long as irreparable harm will not result you're expected to go along rather than make the cop use force to get compliance--resolve it in court, not on the street. It's specifically to prevent resistance by those who think they know the law. (And almost always are wrong.)
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There's no need for such a law because it's implicit in the owner of the property having the right to tell someone to leave.
This isn't like kids on your lawn, Loren, and you know it.
It's United's plane. They can order off anyone they want to. Those who refuse to leave when ordered off are trespassing.