I could understand offering this "benefit" if it really mattered much, but this seems a bit disgustingly class based as passengers ready to exit are not really going to hold anyone up. I do wonder if it's legal - there are very few situations where it is legal to prevent freedom of movement like that. I certainly wouldn't allow myself to be detained from moving freely to where I wanted to go in many other situations (I certainly wasn't going to make a big deal about this on the aircraft in this day and age).