There's a general lack of civility lately, and I blame the parents. I know my parents were very strict and my generation may have "let up on the gas" a little. Spanking (which was allowed in schools!! when I went) has disappeared as a tool, and the practice of having a parent around at home is gone. We had working mothers in my childhood, but a non-working mother always stepped in to cover. That sense of community translates into respect for other and other things.
Maybe the sign should say "Pick up after your pet or you'll get a time out".