At least if you made a mistake, no-one would know. It reminds me of an incident at school when some bright sparks wrote 'What we want is Watnenys' (a well known advertising slogan at the time) in white paint on the cricket pavilion roof.
The Beak was so enraged he threatened to keep the whole school in for an hour's detention if the culprits didn't own up. He carried out his threat the following week and we all reckoned the guilty parties were too ashamed to turn themselves in because they would have had to admit they couldn't spell 'Watneys'!
Although the offending words could mainly be seen from the school, they were also visible from the top deck of passing busses. To spare the reputation of the school's English department, the whole roof had to be painted over in red paint.