You can scroll into them from the outside, the limit isn't set unless you spawn inside of it. This allows for multiple in the same map and also allows none to be set if you don't spawn inside any. From the perspective of what the engine allows, yes you can walk outside of the limit you spawned in and the camera will not scroll with the player. However, this is something that should be prevented using map design. As for permanence, right now they are permanent. I could think about a way to make them expand, but that's not really their intent (at least I don't think I've ever seen a Zelda game do this, please correct me if I'm wrong xD). They would be used to keep the camera from scrolling passed the boundaries of the map to the point where there are no tiles, or for an indoor map. Since all the house interiors in the castle town are in the same map file, this keeps the player from seeing the adjacent house when walking to the wall.