The analogy makes perfect sense, although I guess if I have to explain it's not a very good analogy.
If killing a spider is your goal, there are many ways you can go about it including using your handy shotgun. However the shotgun will have all sorts of unitended consequences, like destroying the surface the spider was on, leaving a bad smell in your room, disturbing the neighbors, etc.
The current system we're talking about is the same thing. Sure, it accomplishes the goal. But it has a ton of terrible consequences as a result. Caravans can't complete their routes. Caravans have the ability to block their own units. Pioneers, 3 dudes with a pick axes, can block opposing Sovereigns, essentially magical gods, from exploring simply by parking their fat ass in any one square wide area of passable terrain (including terrain they don't own).
The system as is definitely lets you have a walled kingdom and block your territory, but it is a shitty solution (if it is intended as the solution, which I don't think it is hence the [BUG] listing) and sends problems throughout the rest of the game. That's the point of the analogy.
Edit: I was responding to Bellack in Post #26 saying that the analogy made no sense. I left this despite the fact that we've come to some agreement, because I still think the point still stands. If the current system is intended, it's terrible. If not, then it's a BUG.