I can't answer the first part: why doesn't it light the A/C light? But you
already know the second part - if they don't always dehumidify the air
blowing on the windshield and it should suddenly fog your view, their
liability is too great. I can't think of a single car made in recent years
that doesn't behave that way. Even my '85 Volvo does that. But if you think
about it, if you are trying to remove frost/ice any humidity in the air will
fog (or frost) the windshield as it hits the cold glass, and if you aren't
trying to defog the windshield you need dry air to do that.

Most of the systems also turn off the compressor if the outside temperature
is below a certain temperature, like 50F, because the air is already quite
dry. Maybe that has some bearing on the A/C light status, because trying to
report what is actually happening could be convoluted. "You didn't ask for
the A/C to be on but it is on because you have the defroster turned on but
the compressor isn't running because it is too cold outside." In the end, we
just keep driving unless the windshield fogs up.

Mike

Sorry, but big brother (or sister) has determined that decision is no
longer an option with common consumers as they are generally to dumb(ed
down) to make rational decisions.

Now, if you believe that you are truly not part of the sheeple masses,
you can buy a pair of wire cutters, hack saw and other assorted "tool"
of modification and rewire/reroute/redesign the system to suit.  Not a
move that I would recommend though...

JT