Is it illegal to snap a photograph of a clerk inside a post office?

Today, in California, I tried to exchange a book of 32 cent
stamps for current stamps, which is something that I have been doing for
decades (whenever they change the prices) at a value-for-value
rate (lately I've been exchanging for "forever stamps" at the
current rate).

In the past, many times, I merely placed the stamps on a sheet
of paper, in the presence of the clerk, thereby making them
unstamped, but still valid.

However, after watching me, the clerk would not allow the
exchange, citing an unwritten "new regulation" that disallows
this common practice. I snapped photos of the transaction, in the
clear presence of everyone (there were about 3 clerks present),
and there was absolutely no protest (it as a large Nikon SLR
which I happened to have hanging on my neck as I was returning
from an outing and stopped at the Post Office by way of errand).

Later, I called the Officer in Charge of that Post Office, who
indignantly said I was breaking the law by taking a picture of
the clerk "without permission".

I must restate, it was clear as the sun shines that I was snapping
pictures, and NOBODY raised a word of protest - but - I must also
ask if there is ANY legal standing for the OIC's presumption that
I am guilty of breaking the law for doing so.

May I ask:
Legally, in California, in a Post Office, when nobody objects,
is it against the law to snap a photograph or otherwise record
the transaction?