Oddly enough it was this that started me out with the idea of window cleaning.
Many years ago I was off work ill. I was lying in the bath when I heard the window cleaner arrive. Lo and behold, he didn't touch the bathroom window. Thinking he'd possibly made a mistake, I opened it and ran my finger along the outside. It was black; it'd clearly never been touched.
When he came for his cash, I mentioned it and he said that he must have made a mistake. He popped round the back with his ladder and did it.
He was deeply offended when I told him never to come back again.
So, if you want my answer to your original question, it's sheer fraud that makes them miss frosted glass.
Vin