The point is that after boarding, as in this incident, passengers can't be removed solely because the airline desires to seat their employees.
Says who? An airline can do what it likes; it's the repercussions that should cause it to think twice.
But you are right, I had missed the point about the incident arising because they wanted the seats for crew. Even more inexcusable, and I stick by my post that they should simply have upped the ante until volunteers appeared. If the choice is that or a charter for the crew it's a no-brainer.
And even if over-booking was not the problem in this instance, it's still a pernicious practice which should be stamped on by the regulators. I know there will always be no-shows, but no money is lost because of them. If a carrier wants to gamble and overbook anyway, think of the excess revenue against the occasional need to spend $1,000s to "volunteers" to get them out of the hole they've dug.
I get the impression that they simply ordered these people off without any
good offer being tried first, and assaulted them when they objected. But I suppose someone will tell me I'm wrong about that.
Let's not forget, too, that this incident is in the context of the accelerating tendancy of passenger contact staff in the air and on the ground to believe that their role is a disciplinary one, combining a contempt for passengers with abusing them at every opportunity. Of course, as air travel has become relatively cheaper to the point of stupidity, so have passengers become more badly-behaved and disruptive.
This leads me inexorably to the conclusion that the cure lies in charging much higher prices, and paying a lot more for good passenger contact staff. But I don't expect PPRuNe to agree.