Japan hasn't filed an objection, hence they're violating the IWC. The hunters, who don't represent the government or its intentions, are in direct violation of their countries agreement.
People on the show are doing what they can to enforce the agreement, and aside from illegally boarding ships (which this show does not do), they are within their legal right to obstruct commercial whaling.
actually thats not true
there is no legal ground to the ICW so its not as if these sea shepards are preventing illegal acts
its been proven in the US where hunters were harrassed on public land by anti-hunting activists. the courts found in favor of the hunters as they were abiding by all the pertainable LAWS. nothing coming from the ICW is a law that nations can be held to
if you are talking in relation to hunters who are going out and fishing/whaling without any sort of permit from their country then thats poaching and completely different discussion.