When the Rainbow Warrior was scuttled by the French, (
http://en.wikipedia.org/wiki/Sinking...ainbow_Warrior) it created a firestorm in the environmental activist community. This included Paul Watson and the "Sea Shepherd Society." Watson was operating the M/V Sea Shephard II at the time to carry out his maritime protests.
The focus of the environmentalists in the Pacific was not whaling, but nuclear testing. They vowed to do whatever they had to to stop it. Now these organizations rely on private donations to operate. They have to walk a fine line between effectiveness and lawlessness to keep in the media and keep the money flowing. Watson continuously pushes that line. Consequently, most mainstream environmental organizations distance themselves. It's all tied to the money. Watson has benefactors, however, that have allowed him to operate.
The UN sanctions and Intenational Law of the Sea Conventions that address whaling aren't signed by the remaining principal whaling nations: Japan, Korea, Norway and Finland. There is still a demand for whale and whale bi-product. On the open seas, unilateral law enforcement is impermissible by treaty. Only the sovereign under whose flag the ship operates can enforce international law, or grant permission to another state to enforce the law, against their flagged ships. The US can only enforce it's Marine Mammal Protection Act within the US Exclusive Economic Zone (EEZ or 200nm boundary).
Folks like Watson are very careful not to violate US law. This gives them access to US ports and services without restriction. Harassing a Japanese ship on the high seas is a Japanese problem. Japan has no navy, to speak of (by peace treaty terms at the end of WWII). So they really don't have the platforms to protect their commercial shipping.
That's the setup in a nutshell. Probably more than anybody cares to know, I guess, but it's important to understand how folks like Watson get away with what they do (whether you consider it wrong or not).
If you want to know why we think they're dangerous, have a look at their "official" history:
http://www.seashepherd.org/whales/se...d-history.html