>>>Here are a couple I think you'll like. I ride the commuter train to and from work (it's called the "GO" train). And every trip (every station, in fact) I am treated to the following announcement: "Remember, it is prohibited and illegal to cross the tracks".
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>>Isn't it a valid sentence? Things can be prohibited and not illegal, like my mother prohibited me to play soccer until I finished my homework. In the same sense, the transit company can prohibit you to do something in their installations that is not illegal. You might find this interesting, although it is not illegal vs prohibited, it is illegal vs unlawful, which I think is pretty close http://grammar.quickanddirtytips.com/unlawful-versus-illegal.aspx
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>If it's illegal, what's the point in prohibiting it? They might as well tell us that killing another passenger is prohibited and illegal.

From now on, you are prohibited from doing anything illegal. So there! :o)