[qiqitori]

Heh, most commenters here are way off-base IMO, and IMO the real reason is hinted at in the article. Notably, there are categories of adult content that are illegal in the US and UK, but not in Japan. Based on something that very recently happened to someone I know in the UK, I think it is likely that Pixiv was compelled to hand over information on users who were looking at illegal-in-UK stuff. Perhaps Pixiv has gotten tired of doing that.

[cpthammer]

The article is incorrect about categories of adult content being illegal in the US. The cited Miller Test has been used to justify bans of real life photography, but the supreme court has allowed first amendment protections for works of art that objectively do not cause harm by their production, which necessarily includes drawings like those on pixiv.