Almost all municipalities have laws against the posting of these bandit signs. It is just a question if that municipality enforces the laws or not. I had to pay a $300 fine a few years ago for posting these type of signs.
That dating site finds people on craigslist to put up their signs. They register their domain names with foreign enitities and use foreign hosting so authorities have a harder time finding them.
There is an active group of street sign police at causs.org. They have people that will immediately remove these signs and publicize the fact that they consider these signs the same as pollution.