Very good question. And do they think that driving to the car wash and back represents the "green" approach? Let's see - 5 miles there, 5 back - that's 1/2 gallon of gas wasted for each weekly car wash. And the car wash uses water, and some fairly strong soap as well (especially the brushless version). Plus, it costs more to have it washed than it does to do it yourself.
You left out the fact the water will end up in the same place as it does when they wash the car at home, in the streams and oceans, which is what they are bitching about.