Why? Perhaps they are erring on the side of caution, or perhaps they thought the warning signs will provide readers a sense of the city's presence in the face of the vie virus threat. Well, it gives city workers something to do now that all the parks are closed and no one should be out there without a good essential reason. Here's an idea.
Why not print flyers and distribute them to the inmates - a captive audience - at the county jails ordering them to "Stay at Home" and "Shelter in Place?" Maybe they haven't heard about it yet.)