That's true, but at least a small rogue trader would has the option of poaching S&D, and it makes sense too, he is basically fullfilling the role of a smuggler (edit, beaten by Huseby), running demanded goods into an area where its restricted by the sovereign power who attempting to enforce mercantalist policies.
This small time smuggler also has ingame efficiency on his side, because to stop him it will cost the sovereign more than he gains. The landholder has the resources of course to keep crushing all these runners, but it still hurts him in absolute terms and weakens his power making him more vulnerable to actual rival powers.
That is why my original message to these small traders back on the past pages was don't be scared by threats of closed borders, go in well armed, and with modest cargoes and take the risk to poach that S&D, you'll have fun doing it, there's a decent chance you won't get caught, and even if you are, the cost of destroying you outweighs the value of protecting S&D so its a material loss for landholders and weakends them vs their real rivals. Resisting trade blockades is not futile.