How do you know this? Where I live it's legal to place advertizing on public property as long as it doesn't permantly deface the property. Thus you see clutters of signs by the side of the road, and even an occasional magnetic sign on metal poles and such, though those dont' seem to stay for long.
There are regulations here about all outdoor advertising. You need a permit under state law, and the city puts all kinds of size, shape, height, etc. restrictions on it, varying by neighborhood. They also placed one in a public transportation station, and for that you not only need approval, you have to pay their rates.