Cabin Crew don't allocate seats, people enter with boarding passes. The CC does not have the right to move someone without reason but does have the obligation to ensure that competent and compliant people are seated at emergency exits. Some airlines now charge a premium for the emergency exit row, a practice that should never have been allowed to come into existence and one that needs to be banned. Try telling the rather large gentleman who paid extra that he now has to sit somewhere else because he is too fat to exit the aircraft quickly. Identifying crew from your or other airlines at the point of issuing boarding passes and seating them at the exits is the cheapest insurance an airline will ever get. With on line check-in and self check-in, this practice is much harder to implement and so probably no longer an option. I've only ever seen Air Niugini and MBA do this anyway, It was probably something that came from Ansett.