Sure it is. Look up the rights you LOSE when on a plane. there plane, there rules. they asked him to remove himself from plane... He did Not, so they did it for him.
And they will be paying out big bucks thanks to corporate stupidity and greed. The guy was basically assaulted not unlike a common street mugging. He is currently scheduled for surgery and has retained the services of an excellent Chicago law firm. United has already announced they are refunding ticket prices for all 70 passengers on the plane, and no doubt some of those will also retain legal counsel.
I've never seen a situation where this entire "ask for volunteers to give up a seat" process was done AFTER the passengers had all been boarded. The normal method in my experience is to ask for volunteers prior to boarding, and to up the ante until they get the number of volunteers they need. Of course they didn't do that this time because those four crew members didn't ask for seats until the very last minute. Stupid on their part, stupid on the airlines part, and really stupid for the local yokels to get the airport rent-a-cops involved. The experts here in Chicago have already confirmed this was an improper use of the airport authority "cops".
I hope the stink from this is strong enough and lasts long enough for the weasels in DC to fix the laws and restrict airlines from this sort of hooray for them and screw the public practice. And they need to extend the changes to the usual overbooking (which didn't apply here) where they knowingly sell more tickets to a flight than they have available. Let them make up the profits on no-shows some other way, like charging those no-shows directly. Kind of like when you make an appointment with a medical professional and then don't show up. The airlines need to be prohibited from screwing over people who bought tickets in good faith and have plans that being bumped will destroy.
Jerry