Basically because NipponDenso engineers are some kind of idiots.

Shortly after A/C became common in automobiles some bright young engineer realized that in the process of the A/C cooling the incoming airflow it oftentimes also dehumidified it. And that of course helps prevent and remove condensation from the interior windshield surface. So from that time on, until about the mid-eighties, it became pretty standard to use the A/C, supplemented by lots of heat, whenever the defrost/defog/demist function was activated at least as long as the A/C was functionally effective, usually only above about 50 degrees F.

Then some idiot decided that if a little bit of something was good then a LOT would be excellent!

NOT!!!

If you wish to use the A/C below 50F (when the RH is usually below 50% anyway, with a few regional exceptions)to dehumidify the airflow then the system must become SUPER EFFICIENT.

The A/C evaporator, the device that does ALL the work, cooling and dehumidifying, cannot be chilled below freezing. So that leaves very few ways to increase its efficiency, my 92 LS400 evaporator is extremely dense and complex, over 10,000 square inches of surface area. The longer the air molecules take to move through the evaporator the colder they become, so in order for the system to work the airflow must be very low.

The entire Lexus product line uses the A/C system EXCLUSIVELY to help prevent and remove condensation from the interior surface of the windshield in defrost/defog/demist mode, a system which is entirely NON-FUNCTIONAL for this purpose with outside temperatures near, at, or below freezing.

So if you hear of someone inexplicably leaving the roadbed and being killed on a snowy and cold day driving a Lexus or Toyota think about this. Maybe s/he lost sight of the roadbed because his/her windshield fogged over virtually instantaneously.

And now along comes toxic mold to further muddy up the issue. Prior to the mid-eighties there was no reason to run an A/C below about 50F, so heat was used exclusively heat the windshield and evaporate condensation from its surface and to lower the relative humidity.

Above these (50F) ambient temperatures when the A/C was used what moisture that was condensed onto the evaporator vanes but didn't run out the drain tube would evaporate away fairly quickly and not cause a problem.

Remember that extremely dense and complex A/C evaporator in my 92 LS400? It came from the factory coated with a porous nylon film into which was embedded an ant-microbial substance.

In the summer of 1991 Lexus already knew that their use of the A/C in already cool climates for these purposes was creating an environment, cool, damp, dank, that encouraged mold and mildew growth.

Look around on the internet at just how many products that have been developed in the last ten years or so to combat the "gym sock syndrome", the mold and mildew smell we all now have to live with because some idiot engineer decided to try a new gimmick.

Remove you A/C compressor relay when the ambient temperature fall below an average of 50F. And turn up the system heat when you activate defrost/defog/demist mode. The response you get for removing condensation from the interior surface of the windshield will be one damnsight quicker than the A/C can EVER be, and it will continue to work that way all the way down to sub-freezing levels.

AND you won't have to worry about toxic mold.