But, we're talking about bumping passengers off a planes that is already full. The passenger already for a ticket on a plane that is full. Now, they're getting kicked out because of pure greed to make more money by the airline. I'd still be pissed.
It's like Jerry seinfeld said. "You know how to take a reservation, but don't know how to hold a reservation."
No, it isn't. Ticket prices fluctuate incredibly wildly, and tend to increase sharply in the month leading up to the flight. In fact, this move is largely to displace overbooking to a new mechanic.
Now why this is a problem: Want to book a necessary flight 3 months ahead of time, shop for the cheapest fare, amd then find out a week before your trip that you've been bumped to a later flight (posssibly missing scheduled activities) because a last minute customer wants your seat? Unoversal adoption of United's model would make this common place (at least as common-place as overbooking is now).