I guess US airlines have less regards for their Pax than in Europe. As I have seen the exact opposite happen, Crews been left behind.
This stinks bad management by the company, if you can't foresee this problem in advance than you should find another job.
UA in this case should have offered more money until they got the seats they needed. You can have a business man missing a million dollar deal because suddenly the airline decides they want his ticket back.
How will cover such consequential loss suffered by passengers how have paid their seat, and have been allowed to board?
As long as they follow the rules, that seat belongs to the passengers, otherwise airlines would not be offering money to get the seats back.
Regardless does not justify to remove an embarked passenger this barbaric way.