Let's say that United is within its legal rights to deplane anyone they want - which due to archaic anti-consumer laws that hark back to the middle of the last century and current spineless lawmakers actually means, for this industry only, that such laws still hold sway.
Does that matter? United has replaced Pepsi as the internets whipping-boy and tanked its shares, compounded by one of the most inept reactions from a chief exec which is virtually certain to end up in a future "worst business blunders" book.
Sometimes being legally right just isn't enough, especially when you're in the legal right because inept lawmakers don't have the courage to legislate your greedy-don't-give-a-toss-for-the-customer ... into the 21st century.
Legally right, morally wrong, ethically bankrupt. That's today's United Airlines.