United messed up and now people are going to tend to give jerk passengers the benefit of the doubt.
I HIGHLY doubt that someone was sleeping across their 2 assigned seats before takeoff. Flight attendants will generally tell you to take your assigned seat first, then move (if there is room) after the flight is at altitude. The courteous thing to do is to ask the attendant for permission to switch. The attendant may or may not grant permission. Usually, they will.
I cannot stand obnoxious and belligerent rule breakers when I travel. Follow the rules or get off the plane.
I can almost guarantee that the couple moved from economy to “economy plus”. Seats have a little more leg room. You have to pay for it unless you have frequent flyer status.