The AC acts as a dehumidifier when it's cold out. That's why it gets turned on. You want to hit your fogged up windows with warm, DRY, air to defrost-defog them.
That's why you can see your breath when you breathe on a cold window: water molecules in your breath. With no AC, your defroster would do the same thing, and you wouldn't be able to see.
Well ok, after a while, it heats up the glass in the window, and you can see, but it takes a lot longer.
<-hasn't had AC in years.