Owners learned quickly to just pull the key back some in the switch when it went off----Then later close and lock the door after they forgot about it!! The smart kids learned quickly to disconnect the pink/black wire from the relay.
It was part of a "Theft deterrent system" campaign on the part of GM for 1968 model vehicles. Big print on the owners manual facing pages and owners envelope about it. No doubt a federally mandated program to remind neglectful owners to be more diligent about taking their keys with them!