They hadn't done anything wrong. When I fish a creek, I may have worms and roe with me. If you had approached me and directed me to the sign, and I went over to read it, I would have still fished.....I always have an assortment of jigs and flys...as well as pliers to pinch the barbs.
The fact that they were surprised and seemed concerned, and the fact that you didn't witness anything wrong leads me to say to give them the benefit of the doubt.