My guess is that it has to do more with FA's not wanting their carts or other passengers to constantly bumping overweight American fliers who start spilling out into the rows once all of the aisle armrests go up. I'm not sure why CO bought seats that do this. There also may be times when someone needs to move quickly up the row for safety sakes and people not using the armrest would be in the way.

That said, I have no idea why you would want to put up that arm rest and how that would lead to a move comfortable flight.