Because I would rather patronize those establishments who don't care if I'm carrying in the first place. Besides I would rather not have to be the one that finds out how people will feel about me ignoring the business owners wishes if I were to be involved in an SD situation behind one of those signs.
In a true justifiable SD situation, quite frankly I personally don't care what signs are posted. Keep it concealed and don't worry about it unless your life is threatened and you are forced to use it. Whatever happens after just happens. I'll deal with it then.