Tickets = Revenue.
Back in the 90's there was this retired lady. Her husband had passed, left her very comfortable. She decided to become a good Samaritan. She got a big back of change and walked around San Francisco, a city with notoriously onerous parking enforcement, filling expired meters for people. Just to be nice. The ******* cops arrested her for creating a public nuisance. The goddam pigs arrested this sweet old lady for trying to brighten up some hard working people's day. But she was costing the city thousands of dollars in ticket revenue.
EDIT:
My mistake. It started in Cincinnati
http://articles.latimes.com/1996-11-10/news/mn-63253_1_parking-meters
Figures. Goddam Bengals.
I think the lady in San Fran got the idea from her, because I don't thing Silvia Slayton did this full time.