There was an article a while back that the signs are placed there for the audience, especially those which are in sight of the camera. People aren't allowed to bring in their own signs.
This is not unusual at all. Most major campaigns do this to control the image (versus having 200 different colors and misspelled signs waiving in front of everyone).
I doubt he brought his own sign unless it was something he had rolled up in his jacket to sneak it in, which may have been what caused the panic. Guy pulling something out of his jacket, people can't tell what it is, someone screams gun, the rest is history.
