Heya...I have traveled pretty extensively in Asia and have hit Japan a few times. Indeed, it does kinda seem surprising.
Still, from my man-on-the-street observations, it really is not so much that the Japanese [and Asian people in general] don't like to read. More, I think, in large part because this part of the world has enjoyed such superior cell phone gear for so long - people have been using smartphones for ages here due to cell network infrastructure superiority versus the USA. So long in fact, that now most Asians use their smartphones for pretty much everything, including ereading. Dedicated devices like ereaders might actually seem a bit superfluous to people around these parts.
Dunno.
Still, I recall reading some stuff here about Sony perhaps making a big localized push into the ereader/ebook market in Japan/Asia so who knows?
And also I think for Americans at least, the ubiquitous nature of giant brick and mortar bookstore chains like B&N and Borders and of course the enormously influential digital presence of the American company Amazon, combined with relatively crappy cell phones/cell infrastructure [leading to slower widespread adoption of smartphones] might have something to do with why dedicated ereader devices are more "popular" in the West.
Again, who knows?
Interesting question though