The issue here is not the security guard being an idiot, but the fact most GA pilots think the rules don't apply to them.
If you are going airside, then you aren't allowed to carry anything. That doesn't change whether you are airline pax or GA pilot.
You could easily carry something naughty airside and leave it for someone else, or plant a device etc.etc.etc.
Is it daft? Yes, but it isn't the guards fault. They aren't allowed to bend the rules. Whilst it has casued me no end of problems in the past, it's no good being a belligerent a*se about it since they have no power to do anything else.
The system has holes in it the size of the Mont Blanc tunnel, but it's no good getting all stressed with the guards. They neither understand or care about GA.