Turning on and off defrost turns on heater

Because A/C has a dehumidifying effect, and less-humid air defogs the windshield faster, selecting a mode that involves defogging the windshield does enable A/C.

A humidity sensor is included in gen 2. (Gen 3, for what it's worth, doesn't include a humidity sensor, but still enables A/C for defogging. The humidity to fog the windshield came from somewhere.)

Back when I was in driver training, pretty much all cars I'd seen had strictly manual controls (and air conditioning was something only other people's cars had). Nonetheless, we were trained to turn the A/C on when using defog, and I remembered and did so manually years later when I had my first car with A/C.

In my gen 3, I also turn off ECO mode when I want to defog. ECO mode suppresses the 700-ish watt electric supplemental heat. Normally, that doesn't make enough difference for me to feel, so I leave ECO mode on most of the time. But it does make a noticeable difference in how fast the windshield is defogged.