Sea Shepherd is acting lawfully under the United Nation's World Charter for Nature, which allows for the enforcement of international conservation law by non-governmental organizations in international jurisdictions. According to their web site:
Sea Shepherd campaigns are guided by the United Nations World Charter for Nature. Sections 21-24 of the Charter provides authority to individuals to act on behalf of and enforce international conservation laws.
Sea Shepherd cooperates fully with all international law enforcement agencies and its enforcement activities complying with standard practices of law and policing enforcement.
Sea Shepherd adheres to the utilization of non-violent principles in the course of all actions and has taken a standard against violence in the protection of the oceans.
Sea Shepherd Conservation Society respects and acts in accordance with the following international treaties, declarations, conventions, and charters:
The World Charter for Nature
U.N. Doc. A/37/51 (1982)
The International Whaling Commission (IWC)
Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES)
March 3rd, 1973, Washington, D.C.
United Nations Convention on the Law of the Sea
December 10th, 1982, Montego Bay
The Convention for the Protection of the World Cultural and Natural Heritage
November 23rd, 1972, Paris, France
The Convention of Antarctic Marine Living Resources (CCAMLR)
The Northwest Atlantic Fisheries Convention (NAFO)
International Convention for Conservation of Atlantic Tuna (ICCAT)
The Declaration of the United Nations Conference on the Human Environment
June 5th -16th, 1972, Stockholm, Sweden
ASEAN Agreement on the Conservation of Nature and Natural Resources
July 9th, 1985, Kuala Lumpur
The Berne Convention
Convention on the Conservation of Migratory Species
The Japanese and their American enablers are the only pirates in this picture.