Well I am new to these discussions and am not an economist so I may be missing something. But this seems again to assume that other routes are available. I'm guessing real-world privately-owned toll roads do not drive their prices up because there are other public routes required by the you-know-what.

In @biophil's scenario, if I understand correctly, we can first suppose that tromping through the (unowned?) jungle instead of the handy river imposes a cost $X. This means people along the river can collectively charge just under $X, right? If any one of those along the river impose a toll that puts the total over $X, then they all lose out; but if the total fees along the river are under $X and one actor lowers their toll by some small amount, that just means another can raise the toll by that small amount and it is still better for the traders to take the river.

Then we suppose that the jungle (and air and sea and underground and ... ) are all owned (is there any reason they wouldn't be in the anarchist utopia?), and I have to say I get confused, so maybe you can help. People along the jungle route realize they can play the same game Ben does. Of course if they charge too much, people will go back to the river - if the river tolls don't add up to be too much. In these kind of rent-seeking scenarios, I take it, each owner may as well charge something, since they are no worse off than when they were letting people through for free if the people decide not to go through at all.

I agree with @discombobulated that the main problem is how to handle private coercion and might making right - and I think @biophil agrees too, in this post. And when it comes to @discombobulated's interesting analogy of world governments, there it seems we are left to settle things by might making right - which I'm inclined to think is unfortunate.