Although it’s pretty terrible, there’s no grounds for a lawsuit because the condition of overbooking and how it is handled is in the contract of carriage (which is the contract between the customer and united) and because the customer disobeyed lawful crew instructions which can actually be a crime. It’s because he resisted that he was dragged off the plane. It reminds me of how much I actually hate flying within the US from being treated like dirt by TSA to unfriendly airline employees.
As for others saying the airline should have offered more than $800 etc this is actually a case of government regulation failure. The maximum allowed by governmental regulation is $800. This is why airlines offer amounts for volunteers before this to just below the maximum. Some airlines also have an overbooking bid process in their check in where you can pick the least compensation you would be willing to take. If that regulation was removed (the regulation benefits the airlines surprise) the airlines would have to raise their offers ever higher until they had enough volunteers.
I fly United quite a bit, I have their top frequent flyer status, and I agree they treat their customers very poorly, especially if you don’t have frequent flyer status with them. They have some of the rudest, unfriendly staff around.