Given that United wasn't the one who dragged him out of his seat and beat him (that was airport security), it's a bit of a stretch to suggest that United should have been liable for this. Most likely United just wanted the lawsuit out of the news, so millions of dollars was cheaper for them than the lost goodwill -- even if they would have ended up winning the suit.
Weird, I didn't know the airport security had any saying in bookings, it's almost like a United staff asked them to take care of an "unruly" passenger.
Airport security might have followed orders but those orders couldn't have come by anyone but United.
