I'll definitely not be flying United or Delta ever again after this incident. Over-selling spaces on a plane is a scummy practice and shokldn't be allowed. When someone needs to cancel they still pocket the change fee or keep tne entire fee if they miss the flight. Seems like borderline fraud.
Overbooking is an extremely common practice. In fact, I don't know of a single airline that doesn't overbook. On a plane with hundreds of seats you almost always get cancellations; the small amount of times you don't, you almost always get people volunteering to be pushed to the next flight in exchange for money (I've done it myself a few times as I don't mind working in airports).
Cases like these are exceedingly rare. There's a million fucked up things with how they handled the situation, starting with assaulting the passenger. Overbooking itself is the last thing you should be worried about.