You can't remove a seated passenger from the plane without mutual agreement.
The plane was heavily overbooked. I guess even at check-in or the boarding gate, a number of passengers had been denied boarding, with or without compensation.
From what the news reported, 4 members of a partner airline suddenly turned up (can't imagine why there was no prior communication that these 4 members needed to fly) when the plane was already fully loaded, and all passengers seated.
United then offered cash incentives (increased to US$800) and asked whether any seated passengers would voluntarily give up their seats, no one budged.
And United then "randomly" selected 4 passengers to deplane (rumour said the lowest fare passengers were picked).
The problem of overbooking isn't created by the passengers. The passengers are at no fault.
Again, to me, you should not remove a BOARDED and SEATED passenger (no matter what fare he/she pays) without his/her agreement, let alone BY FORCE.
I had a similar experience when I flew another US airline, from Seattle to Fairbanks. When everyone was seated, the chief gate agent came and offered $1500 to any passenger to deplane, plus a free hotel night and flying in Business Class the next day. No one moved. Everyone chose to fly. The plane left on time.