When you go into a restaurant and you see a sign that says “No Dogs Allowed,” you might think that sign is purely proscriptive: Mr. Restaurant doesn’t like dogs around, so when he built the restaurant he put up that sign.
If that was all that was going on, there would also be a “No Snakes” sign; after all, nobody likes snakes. And a “No Elephants” sign, because they break the chairs when they sit down.
The real reason that sign is there is historical: it is a historical marker that indicates that people used to try to bring their dogs into the restaurant.