I am a state prosecutor so I also get the exception to the prohibition under:
"(d) Subsection (a) shall not apply to -
(1) the lawful performance of official duties by an officer, agent, or employee of the United States, a State, or a political subdivision thereof, who is authorized by law to engage in or supervise the prevention, detection, investigation, or prosecution of any violation of law;"
Nice phrase when you said not giving legal advice. Anything any lawyer does of this nature is for general educational value, not as "advice."
I've got child porn case I'm working with some Postal Inspectors, next time I have the chance I'll ask what the SOP is for response to otherwise lawful firearm carrying.
20 years ago in college I was a contract mail truck driver, the "rules" said no guns. One time I was shot at, blasted the side window out of the truck. That left me with something less than warm feelings for bureaucrats who write these BS regs.