The policy goes back at least to WWII, if not before. Military commanders of installations are given wide latitude in deciding who may be armed on their base. Political correctness has meant that nearly everyone is disarmed on base, as commanders fear to tread on politicial toes because their career may be cut short.
I served in 1960 and everyone was unarmed except Military Police.
The MP’s served as the law on Post at that time and we never saw civilian police.
The Military Police wrote the traffic tickets, directed traffic and did all of the things a civilian force might do.
Political correctness has nothing to do with the policy of securing all weapons and ammunition.
The real reason is the fear that some one will lose a weapon or there will be injuries from accidental discharges. No commander is willing to place his career at risk.