Where does it say the signs were posted anywhere? Or that the dealers themselves made them? The report you're linking to only says that the one sign was sent to the police by someone anonymous, and offers no suggestions as to where it came from. The implication was that it was made by the informant for its shock value, which is consistent with the frustration of "WE all know this is a drug house, how can the police NOT know?"

You know, neighbors not being bound by due process, chain of evidence, and all that trivia.