The Japanese perhaps don't understand environmentalists' tactics:
More than 40 police officers raided our offices and the homes of the
activists, and spent 10 hours seizing cell phones, documents, and
computers, despite the fact that we had documented every step of how we
obtained the whale meat, turned the full dossier over with the
evidence, and made ourselves available to police to help with the
investigation at any time. A simple phone call could have brought
Junichi and Toru to the police station. Instead, the government made a
public spectacle of shutting Greenpeace down
This was in response to apparently uncovering a whale meat scandal, according to Greenpeace. The Government says it is due to them stealing a box of whale meat.
It is possible that the Japanese government is heartily sick of the trouble called by Greenpeace and has lashed out. In that case it is certainly an overreaction and regrettable. But it couldn't happen to a nicer bunch of activists.
Just because Greenpeace documented how they stole the box doesn't make it legal. They should face charges. However there is surely no excuse for holding someone for 3 weeks without charge - I note that the investigation into their claims finished very quickly. Even Greenpeace activists have rights. They should be freed.