Mistakes happen. They may have had a last minute order to get crew onboard. If they need the seat, they’ll get it.
Like I said, it’s unfortunate but that is what happened. That’s why you are compensated with a nice check and confirmed seat.
Here ya go:
http://nypost.com/2017/04/10/officer-who-dragged-bloodied-passenger-off-flight-placed-on-leave/