There is no treaty. You can keep calling it that to justify your position, but it doesn't change the fact that the IWC is a voluntary organization with voluntary membership - with absolutely no authority to enforce anything that comes out of the organization.
You slam the Japanese for participating - and voluntarily abiding by the IWC imposed quota system that regulates the various species. What about Norway and Iceland? Norway was a member of the IWC that objected to the moratorium and research quotas, so they are not obligated to abide by them at all. Iceland said they'd join the IWC if they wouldn't apply to them either and the IWC let them join.
The IWC was established to:
provide for the proper conservation of whale stocks and thus make possible the commercial whaling and the orderly development of the whaling industry
The US has its own loophole that we exploit.
If they close all the loopholes, they'll just start a new whaling society to monitor whale populations and determine what hunting levels are sustainable - and all the countries involved with whaling will join the new group.
The IWC is like the Rotary Club or any other club. They are interested in gathering dues paying members, but have no authority to impose their own laws on others (or to claim ownership of, or the right to regulate, vast sections of international waters)
The Sea Shepherds are claiming to enforce a law that doesn't exist.