I used to do that sort of thing everyday 20 yrs ago in central London ,regents street ,and all around there.
used to enjoy the danger ,never felt unsafe,i used to feel more venerable on ladders ,walking ledges was fine.
bit extreme I know ,and wouldn't do it now . 
So did I, 45 years ago
It was the accepted method of doing the job and all 'tumblers' were trained to do it that way. As Jimiwindows says: if you didn't have the bottle you were soon out on your ear.
Even though there were very few accidents and even fewer fatal ones, there were still accidents. In 1978 a window sill collapsed while I was leaning on it to reach the upper panes; it had been broken and repaired by skimming over the damage with sand and cement and gave way under the pressure of my hand - I lost my balance and fell 6mt from my ladder onto concrete. I still walk with a limp and both feet give me gyp at the end of the day.
Even further back in time small boys were forced up chimneys to clean them.
Times change - practices like that belong in the past.
And a good thing too!
On the other hand - if that man had been working over a private garden and there were no people below for him to fall on/drop his squeegee on etc, shouldn't he have the right, as an adult, to decide for himself whether he wanted to take that risk?