Is it really illegal to snap a picture of a clerk in a Post Office?

In article , Eric Stevens
wrote:

When in public spaces where you are lawfully present you have the right to
photograph anything that is in plain view.
When you are on private property, the property owner may set rules about the
taking of photographs.

I seem to the question of your subject line was debated at length some
time ago and the conclusion that was eventually reached was that:

1. A Post Office is a space to which the public may have access but
it is not a 'public space' in the strict sense of that term.

2. The post office and its premises are the subject of acts and
regulations which define what should or should not be done.

3. Photographs may not be taken within a post office of the staff or
the premises without the permission of the post master.

I expect someone will correct me if I am wrong.

that might be true where you live but it isn't applicable to the usa,
where the post to which you are replying is referring.

for usa post offices:
https://about.usps.com/postal-bullet.../html/info_005.
html
Informal snapshots from handheld cameras for personal use may be
allowed at the postmaster¹s discretion provided that there is no
disruption to Postal Service operations and that the pictures are
taken from areas accessible to the public. In these cases, no prior
permission is required from the Office of Rights and Permissions;
however, no lighting or scaffolding may be set up, and no picture can
depict any Postal Service employee, customer, security camera, or
cover of mail (i.e., the exterior of a mailpiece, which would show
customer name and address among other things). Postmasters may
restrict any and all photography if they determine that it is
disruptive or there are potential security concerns.