Skateboarding on the slope of the overpass can be hazardous to the skateboarders and others, but this is not a well-worded sign -- it might better say "no skateboarding." As worded, it would require skateboarders to cross the busy street below the overpass rather than only to walk over it while carrying their skateboards. Latest I know, possession of a skateboard is not against the law.
Someone else clearly wasn't happy with the sign, but it has been painted over. There's no telling what that person wrote -- probably for the better!