Another question that could have been resolved by actually reading the owners manual.
Defrosting your windows is a two-fold job. Not only does heat help to dissipate the fog, but you need to dry the air. The evaporator (part of the AC
system) gets cold, which causes humidity to condense on it then drain away, effectively removing excess moisture from the air. So the AC
automatically turns on even in the winter, to help dry the air so that the windows don't fog up.
Secondly, the reliability of your AC
system would be very poor if it sat for 3-6 months at a time without running. It relies on the pump running to circulate lubricant around the system. If the system sits too long without running, then the lubricant drains off the seals and other parts. When the system gets turned on again, the seals will run for a few seconds without any lubrication. This can destroy the seals and cause the system to start leaking. Also, AC
systems are not completely airtight. There are tiny pores that the refrigerant can seep through. The oil molecules are too large to pass through these pores, so the lubricant can actually seal the pores. If the system sits too long, the oil will drain away from those parts too, allowing the refrigerant to escape.
Probably more information than you wanted to know, but now you know why your AC
does and should run in the winter.