The Captain isn't the person who decided to bump 4 paying passengers in order to relocate some United crew. Whoever made that decision was just trying to take the cheap way out, and needless to say it backfired. So far this mess has cost the stockholders of United more than 500 million dollars, and undid the effects of millions of dollars of advertising.
For a few hundred more dollars United could have gotten volunteers, but they were too cheap to do that.
And regardless of the history of the passenger that was injured, it is now obvious to everyone that United cares a lot more about its employees than its customers.
The Captain is responsible for everything that happens on the aircraft.
Why has no one mentioned the Captain?