The AC isn't just used for cooling, it's also used for removing humidity. The AC turns on when defrosting, even when blowing warmish air, because it helps take the moisture out of the air to prevent the fog.
This is spot on. It happens more in areas where the weather is humid and a few degrees colder than inside your vehicle (not so much when it's extremely cold).
If you pump humid air into the vehicle your windshield will fog quickly. Air loaded with humidity touches the cold windshield, lowers its temperature and with that goes its ability to retain the moisture, so it condensates all over your windshield. This can be mitigated with antifog products (like Rain-X, etc) to a point.
It happens here all the time: weather feels nice, it's in the 60's outside (but very humid), you decide to drive with the windows open but just a little bit because of noise, and you blow the fan off the upper vents directly to the windshield. Within seconds you start fighting with the fog until you give up, close the windows and run the AC in auto.
The compressor will only kick in when needed, the light is on all the time but the control is automatic. If it bothers you too much, turn the AC off manually and make sure you carry a dry cloth to manually remove the fog from your windshield every 30 seconds. As an alternative, you can put black electric tape over the AC LED and just eliminate the anxiety effect while you let the system manage your humidity the right way.