Sadly also a sign of the times, since in many companies these days staff are no longer authorised to take initiatives based on experience and common sense!
Presumably some bean-counter has worked out that, if there's a long queue behind you at the ticket desk of passengers about to pay the on-the-day fare for the flight you want to swap to, then giving you one of those seats would lose them one of those fares (assuming the flight is now full).
Highly unlikely, I would agree, but then that's how bean-counters work, hence no discretion available to the staff at the coal-face.