Yeah, I don't really know what the logic is behind this, but it's not just the Japan forum that is affected. In fact, it looks like more and more subforums are being created. Now, France, Germany, Italy, etc., have been moved under a single Europe forum.
Personally, I am not a fan of the extra click involved
I'm sure it sounds trivial, but not all places have the greatest internet connections... and this makes it more difficult to see if the forum you're really interested in has any new posts. I hope the change will be reversed.