You are both right of course. It is not a freedom of speech issue and a landowner should be able to do whatever with their own land, which the railroad did. However I do have to agree with the comment in the article Re: had this been an ad for NAACP membership not one word would have been said. In the unlikely event that something was said or the sign removed, lawsuits would have been sure to follow. There are groups one can disrepect with impunity and groups one must revere.. apparently.
The NAACP is a protected class doncha know.