I mean, did you read it? They asked for volunteers and when nobody wanted to give up their seat, they said "okay, we're going to choose four people at random" and this guy was one of them.
This is a PR disaster for United, and after the leggings thing this definitely doesn't help their image any. But this happens with basically all major airlines and is part and parcel with the airline industry. Overbooking happens, people get bumped, and sometimes you gotta hop off the flight and get on the next one. You'll find the same shit happens on Delta, American, Southwest, or any other carrier. People trying to get all indignant saying "I'll never fly United because of this" well, if you want to stay true to your morals have fun driving, because this isn't unique to this one airline.