Why do People Spit on The Heart of Midlothian?

Spitting on the streets is not quite gentlemanly behavior, but on the Royal Mile in Edinburg, it is almost a ritual.

The object of contempt is an innocuous heart-shaped mosaic set in the cobble stones of the street, just to the west of St. Giles Kirk. Known as the Heart of Midlothian, the sign marks the spot where the infamous Old Tolbooth prison once stood. It was the place where the people of Edinburgh gathered for public hangings and spit at the door in disdain for those imprisoned inside.