You raise a good point. In this case, the Japanese are still members of the IWC, no one is forcing them to remain a member.
The membership voted a ban on commercial whaling . If they wish to withdraw from the IWC your point would be valid.
As I understand it, the Japanese claim to be doing research, not commercial whaling. I can't judge whether or not that is so, that they are engaged in commercial whaling, if it is so, they should be condemned or sanctioned in some way by the IWC, not by some group of self styled enforcers who operate outside any rule or law.