I don’t buy class warfare argument, unless those doing the warfare are Marxists.
Once an airline takes a boarding pass from the passenger and lets him get on the plane, it is breach of contract to force the passenger off the plane. That is Capitalism at work.
Deciding that the individual must yield to the good of society is a Marxist idea. So, I guess United Airlines thinking that their employees were more important to society (the smooth operation of the transportation system) than their individual passengers makes them good Marxists after all.