They don't care about the shortest path. I'm not sure they ever will.
Turns out that finding the shortest path through obstacles was really, really hard. I was dreading doing this and dealing with the long-term bug factory is would become until I realized that there is actually no way for a player to wall themselves and all their buildings in (well, there is a way, but I'll let you guys think about what it might be

). So the buildings will always be exposed. If you wall yourself off, I think it's okay for the raiders to:
-First, destroy any threats (turrets etc).
-Second, randomly smash up and light the place on fire
After all, they don't know the layout of your colony so there's no real reason or need for them to find the shortest path. Does that make sense?
Note that none of this applies if there are doors. They still go straight after doors if there is one. Though I may change this and have them do some more broad-based destruction if you're hiding behind closed doors.