There is absolutly nothing in the regulations that state before boarding.  What you normally see with airlines is that this happens before boarding because they know they are overbooked and get that resolved before boarding but the regulations themselves do not stipulate before or after boarding.  This is not an isolated incident to United and people have been booted while on the plane.  Actually United is not even the worst offender in bumping people off planes as I believe its Delta.  In this case they were not overbooked but instead wanted to bump for their own employees.  This is outside of the oversale policy and thus United is not safe under that regulation.

At Bolded:  Now you hit the right question.  This is why you elect officials to protect your rights because of situations like this.  The regulations could change if enough outcry but currently there isn't anything preventing any airline kicking you off if they oversold their flight.  This only pretains to the oversale regulations as I have not seen anything outside of that regulation on the topic.

Edit:  Here are your policy makers to the rescue.  As I stated there isn't anything preventing Airlines from bumping you either before or after you are on a plane but your trusty congressmen are looking to add a bill to remedy that problem.

http://www.wbaltv.com/article/bill-would-bar-airlines-from-forcibly-removing-passengers-for-overbooking/9267055

overbooking is not a problem if handled correctly...
its like taking a risk... and if you lose, you pay...
so, best, easiest and cheapest solution in this case (already boarded):
increase bribe... everyone has its price.
or book your employees on other planes... or get them somehow else there

It's going to be a fucking problem now for these airlines because consumers are now very much aware of this issue and are getting pissed off about it, and that means their local politicans are going to be getting an earful about it. 

If an airline was smart they would start advertising heavily that they don't overbook to shit on the companies that do. 

Have lesser means failed? Have lesser means been deemed unsafe to attempt? Was force used as a last resort or wantonly and maliciously?

A lot of questions that will be brought up in court will probably focus on the above and more. Did the police use the least amount of force reasonably perceived by a reasonable police officer? The policy of the airline is irrelevant to the actions of the police right or wrong. The airline will probably lose in civil court and I agree at this point that this probably could have been avoided by other means. As a public servant, we have to remember we are not meant to be supreme overlords of the public. We are supposed to serve the public.

Even in that rule it cites (https://www.law.cornell.edu/cfr/text/14/part-250) it only talks about being denied boarding. He wasn't denied boarding. If the rules are about how you can be denied to board the plane if you have a ticket (but not a boarding pass yet), you would assume that that means before boarding. If it has been happening more often that people have been asked to leave the plane after boarding than that was outside regulations as well.

Anyway good thing more specific rules are being drafted now. Still disgusting that it has to come to this to knock some comon sense into companies. Good name for that bill too, Customers Not Cargo Act.

So the CEO, that f**ker, thanked the staff for their great work the other day and now that the share price is dropping he is horrified? What a sack of shit. United Airlines has to pay for this somehow.

Most of these posts I see are prejudiced against United Airlines but the fair and just view would've been this:

Under the code of federal regulations, title 14, chapter 1, subchapter F, part 91, rule 91.11 in subpart A, according to the Federal Aviation Administration it says, "No person may assault, threaten, intimidate, or interfere with a crewmember in the performance of the crewmember's duties aboard an aircraft being operated." 

Most of the people saying you can't deny service to a man after boarding are dead wrong as crewmember's can and WILL pick off and choose to remove people. The staff was well within their right to call security to remove the man. What is potentially wrong is security using excessive force but to his credit he got suspended.

For anyone else thinking this is going to be a fruitful lawsuit are mostly wrong too, the plaintiff would most likely lose in this case so the best he could hope for is a small settlement outside of court at best. 

Yah it's like you are going to a fency restaurant with your wife/husband (you have a reservation). You take your place and order a meal and some wine. 10 minutes later, the waitress comes to you and asks you to leave, because some employees from the restaurant needs your seats, they are starving... please come back tomorrow...

Boohoo, rules are rules ... 

The point is that United had ZERO right to ask him to exit the plane. When companies overbook, they can block people from boarding, but they can't remove the ones that already boarded. So, if the plane is full and an extra guy tries to board, they may deny it. They can't remove someone so he can board. Also, there was no extra paying passenger: they wanted to fly their workers. And they intended to remove a PAYING customer to do so.

You're also trying to defend them while they removed him with such brutality that he was knocked out. They can't do that. Do you see any of the security guys hurt? No, they are fine. The guy is a bloody mess. It's clear that they used disproportional force to remove an old dude when they had no right to do so. The guy is 69 year old and they could never, ever, hurt him like that. And they tried to argue that a 69-year old man posed such a threat to security guys that were significantly stronger and younger than him so they had to use blunt force. That's the kind of argument I expect from kindergarden childs trying to cover up an obvious mess.

So I really can't see why you're defending the company when they clearly acted wrongly when trying to enforce something they were wrong in the first place in different ways for doing so.