Where in the article does it say that the signs were ON his property?
The way most town lots work around here is you property starts at the curb but the city has an easement right so far back from the curb (here I believe its 8 feet) so its YOUR property but the City can put signs and such in the section. So you can decide if you want POLITICAL signs there or not but if the city wants to put a "caution children playing" sign there you got no say in the matter.