More than likely, the addition of the grounding conductor was due to someone getting electrocuted or injured by an appliance in another industry, such as a clothes dryer, stove, etc. Since the gear in our hobby is considered "appliances", it would make good business (and legal) sense to just move over to 3-conductor power cords just in case, on the 1-in-a-bazillion chance someone got injured touching the metal chassis on an amplifier and there was a fault inside, electrifying the chassis.
That's my guess, anyway.