The sea shepard nutz were flying the jolly roger and had it on their boat’s ‘fins’. Nautically, a ship flying those colors are by definition a pirate. Doubt that the dutch have jurisdiction over that part of the ocean anyway.
Nobody has jurisdiction over that part of the ocean. Australia claims one, but Japan doesn't accept it, for a good reason - you can't claim ownership of such a large space. Besides, an old principle says "you own what you can defend" and no country on this planet can defend that much water.
Nautically, a ship flying those colors are by definition a pirate.
Any ship's captain is the lawman on board of his ship. If pirates approach, flying the flag and attempting to disable his ship, he can use any available methods to defend his ship.
Besides, Japanese ship had a purpose to be there. While some dislike their purpose, it is an internationally approved activity, and Japanese broke no law. However Sea Sh...ds had no legal purpose to be there, unless they are willing to admit that they came to interfere with legal activity of another vessel. Then the court can adjourn right there and then because it's obvious who is at fault here. If Japan files a countersuit, which is likely, then the Sea Sh...ds will be arrested on the spot, for piracy.
They chose the Netherlands simply because it was a neutral third party.