I've heard that car A/C doesn't work below a certain temperature
because if it's freezing out, the coils might freeze up. Yet the guy
at the dealer says, run the A/C to clear interior fog on the windows,
and doesn't make any exception for freezing weather. Could someone
explain what actually happens?

If the A/C doesn't kick in when it is freezing out, how come the A/C
light still lights up?