It could be that they, like most Humans, tend to be nervous when they cannot see. I'm lucky enough to have decent night vision, and actually feel safer when I know I can't be seen. Walking through an area with lots of streetlights makes me feel like a fly on a whitewashed wall, much easier to see and to follow. However, when stepping out of a house into a well-lit street, everybody would find it hard to see into the darker places before their eyes become accustomed (which they don't if there are lots of streetlights) so they would subconsciously avoid the darker bits.
As for graffiti, there's not much point in putting it where it cannot be seen, and where the "artist" cannot see what he is doing.