Warning!!! Opinion coming.
Based on another thread about this very topic... it is perfectly legal to take photos of any location from any PUBLIC place like a sidewalk. Taking photos while standing on PRIVATE property may be trespassing.
Do a search for "The Photographer's Right" as a PDF. It is an American document, but I use it when needed.
Just consider the Papparatzi (sp) aspect... they take photos, as do regular respectable media persons of items such as these.
If there was a break in, or hostage taking or bake sale at the post office... don't you think the local TV station might show the location on the nightly news? Guess what is likely in the background!!! The Post Office???
Chances are the person is another in the misguided "Homeland Security" over kill faction. I would ignore them completely, but you are free to do as you wish.
BQ