It looks like their IP is residential. This ISP even has 1gbps lines commercially available in Tokyo from what I understand. I wouldn't be shocked if just put up a rack in up in the ISPs datacenter to make use of the cheapest bandwidth rates. They probably calculated most of their bandwidth in Japan will be in this network and they have good US peering.

Since, one rack today can be stuffed with 90 servers each up to 20x more powerful than what was around in 2005, I doubt they really require more than 1 well configured rack for a long time. When the time comes to add a second rack they always have the option to place it in another Japanese city, if they don't decide to goto another Asian country instead.

Considering the above, I doubt the savings were worth moving out of Tokyo.