Anyone ever try to sit and pee with a side-arm holstered to your belt?
I have had "neighbors" try and reach under the stall and steal my wallet. I convinced him he didn't need it by stomping on his hand with my riding heel. While the event otherwise ruined my sit down, hearing him squealing as he hit the door was pretty rewarding.
In this case, I imagine if you were carrying a sidearm, you wouldn't want it hanging down where someone could make a grab at it. Very well could be standard procedure to unholster.