@Thomas,
You certainly know how to ask the hard questions, just as a girl is about to leave her keyboard to go out, Thomas!
Whaling was supposedly banned by the IWC in 1986, it's just the "scientific" loophole that has been exploited by Japan, to continue commercial whaling. I don't know that exploiting this loophole exactly makes it "legal-ish". Other countries which aren't signatories to the convention at all & openly engage in whale killing.
As for the "the law of the sea", "piracy", etc ...
incredibly convoluted! We got very bogged down in these questions when the Japanese whalers took some of the SeaShepherd crew hostage (with Japanese government blessing, apparently), around the same time Australia was patrolling the Southern Ocean to keep tabs on Japanese whaling activities ...
then the Japanese government was leaning heavily on the Oz government to take legal action against SeaShepherd for lawless activities against the whalers ... meanwhile SeaShepherd claimed it was acting on/enforcing
Australian law (because the Oz government wasn't!) in protecting whales in the
Australian protected waters (this status is disputed by Japan & others)
.... So you see, Thomas, when it comes to legalities, it's a big, big, grey area. And as you can also see, the recent IWC meeting in Portugal resolved nothing. So I guess, it means business as usual in the Southern ocean this summer!
Whether you see SeaShepherd's activities as "piracy" or not, depends very much on your political perspective. What measures is it reasonable for the likes of SeaShepherd to take, to protect whales from slaughter, given that the moratorium on whaling has never been properly enforced?