A lot of handwritten signs are noticeable on walls all over town, warning people of dogs, and imploring/shaming people not to urinate on the walls or other locations. This sign bridges the two topics, equating a public urinator with a dog.
Public urination/defication is actually a common issue and problem. Driving around town, it is really normal to see men with their back turned to the street, pissing in to the grass or on a wall. Sometimes, their back isn't even turned. Its gross, of course, but its also one of those cultural issues that is easy to look down upon without examining the circumstances which cause the phenomenon (like a lack of public restrooms, running water, or other alternatives).