That's not the point tho. In the street I would agree with you. But this in airplane where security and disturbence is really something very important.
And I said sue as an exemple, filling a complain, w/e... Eveybody can do that yes. Your trust in the system is very limited :) 

Of course my trust in the system is limited, with all of the shit that happens I don't know anyone would.

...Why does US still keeps constitutions around if they don't care?

To make it seem like they care.

Doesn't necessarily mean the law has been broken. The plane is private property, and when the owner of the property is legally able to demand that you leave then there is likely to be a legal defence of reasonable use of force in what amounts to trespass. 

If someone can assert the right to shoot and kill someone for stepping on their property without permission, then surely non-lethal forceable removal from private property is likely to be defencible in court.

In this case the constitution is likely to be on the side of United. Protection of private property rights. When you are on private property your constitutional rights are somewhat reduced.

So, if someone pays rent to live in an apartment, you think that the landlord should be allowed to force him out to let one of his buddies stay, getting the police to beat him up and drag him out?

Depends on the terms of the rental agreement obviously. You'd be kinda dumb to sign such a rental agreement, but if you were domb enough to sign it with that sort of condition, then yeah, the landlord would have a defence for forcibly removing you if they have a lawful basis for demanding your departure.

That's how contracts work, you abide by the terms of the contract or you suffer the consequences. If the terms of the United ticket, which represents a contract for service, says that you can be bumped at any time for specified reasons then you are bound by that contract and United is entitled to enforce it. And if necessary to vacate you from their private property possibly be legally justified in using physical force.

And don;t equate United employees with "buddies". They are employed under contract too, which both the employee and United are obliged to comply, and that will include getting them to where they are required to be to do their job. Or to be delivered back to their home after ending their shift under specific terms. Are you wanting to prefer the passenger's rights over the rights of the conditions for the United workers? 

Should United be a lot more organised in managing passenger and staff logitics? No doubt. But that doesn't change the probably lawful rights United has given itself in tickets.

I think the cops are at fault, not because of what they did but how they did it. Needing to use excessive force on an old man to remove him from a seat shows that these guys are a bunch of incompetent buffons, buffons who are (likely) going to cost the company a lot of money and that old man a bunch of blood. (I mean, I get that shit happens and that things can get frustrating, but c'mon, he was just an old guy, taking a hold of him shouldn't be that difficult for TRAINED PROFESSIONALS, right?)

Also, if anyone is at fault here is the airline, for failing to plan ahead and allowing this shit to go through. Unless the 4 employees thing was a last-second thing in which case; well, I think a better way to resolve that would've been to look for a flight that wasn't already full, I doubt that they needed to get a fly that urgently, but in the case that they did, why not raise the compensation? Instead of $800 do $1000, that fails? Raise it the most you realistically can and THEN you can start using force. Makes sense right? Too much of a delay on the flight? Tough shit, you brought it upon yourself by not planing ahead (This also includes making a bunch of extra seats for such an ocassion).

What you can draw up in contracts is also bound by rules. The same goes for using reasonable force. Otherwise it would be ok for Nintendo to call the swat team on anyone playing botw on Cemu.

We've gone over this before in this thread, no where in the contract of carriage from United does it state that you can be removed from the plane in case an employee needs the seat. The rules for overbooking are all about being denied boarding. As well as the official regulations regarding overbooking. The only rules after boarding are safety/disturbance related.

So it seems the airline called airport security under false pretenses to intimidate the guy into leaving. The guy called their bluff and instead of airport security trying to mediate, they decided to use force. There was no security issue, no disturbance until security started pulling on him. It also seems they never charged him with anything, didn't arrest him, so using force was unjustified. There was no trespassing charge.

After all that, it most likely was unreasonable force as well.
https://www.nij.gov/topics/law-enforcement/officer-safety/use-of-force/pages/welcome.aspx
https://en.wikipedia.org/wiki/Graham_v._Connor
The question is were they trying to arrest him and was he thus actively resisting arrest. Or were they simply impatient and decided to drag him out.