From what I've read in the past, a pilot's weapon must be secured whenever they leave the locked cockpit. The logic is that if the TSA did their job, then a potential hijacker will not have a firearm, so an armed pilot outside of the cockpit then becomes an obvious target to obtain a weapon from. As we know from the past, hijackers often come in groups, and a pilot bum rushed by several guys in the main cabin stands a good chance of being overpowered.

Initially, I believe pilots had to lock their weapon in small gun safes but there were complaints, like with space inside the cockpit. Then they went with these ridiculous holsters.

Like AROTC was alluding to, I think there's a screwy process here that is probably really to blame, with a holster that could allow the padlock shackle to be hooked in front of the trigger rather than behind it. I would tend to believe that explanation more than a commercial pilot was sloppily handling his weapon while the plane was on approach.