I disagree. United chose the other passengers who were to be removed as well, and so none of the chosen passengers volunteered to leave the flight as there was no alternative (default) option presented in which the passengers would remain on the flight. Threatening to call the authorities if someone doesn't leave doesn't qualify as eliciting a voluntary exit.

I'm not arguing for or against, I was explaining what the other poster meant since it seemed as if their meaning wasn't clear.

My statement about the doctor being the only one that could be compensated was definitely tongue in cheek.