Stuart I can see your point, but let me show you mine, and it's a bit more harsh than your vegetable comparison.
Lets compare whale hunting, and the ignorance towards what's going on, with the Holocaust. If both Europe and North America ignored the events going on in concentration camps with thousands (perhaps millions I can't exactly remember) of Jews being murdered, what would our world be like today if the world ignored the holocaust?
Now my point compared to yours is that, if people hadn't taken action to something that wasn't their country's business then more innocent people would've died, so if the world doesn't take action to Japan's business then more innocent whales will die.
Now, after the war the United Nations was formed am I right? And under United Nations laws in which Sea Shepherd crew are part of countries participating in these international laws, and so are Japan. Sea Shepherd are protecting the whales under the United Nations World Charter for Nature, which Japan are obliged to obey to. Nevermind the IWC, that is as you say, just a group that not everyone has to obey to.