So put the adjective "walkable" in front. The question is the same. These things are unwalkable and therefore not able to be part of any route.

Same with your car: the places without roads are undrivable and therefore not available for routing. But with a car, the speeds are so much greater than bipedal motion, shortest vs. fastest is a viable question. When the routing system avoids obstacles, it's mostly going give you the shortest and fastest route. brador makes a good point about grade; with enough grade on the shortest route, it might not be the fastest.