Anything that has to do with defrosting means that you want the humidity lower in the truck's interior than it currently is, the humidity will not get any lower if it's recirculating the same air over and over again, it even runs the AC pump when it's in a defrost vent mode, like most vehicles do. But yeah, you're right, the fact that it doesn't allow you to force it to recirc mode is just dumpy, but maybe there's a way to modify the controls to allow it.