That would seem like a good idea but they’d lose the care and control aspect.
If something is outdated or incorrect it is much more difficult to correct an ebook than it would be to simply change the website. I’ve already received an edit for an ebook here which makes the original ebook ‘incomplete’. Sure, it was an easy fix and simple enough to keep both the ebook and the corrections together, but it’d be much easier to have the bulk of the site in web format and add or correct that.
There would also be a loss of control if this went into ebook form. an ebook is really, really easy to pass around. There are those of us that would pay for content we want but there are also a lot of people that would see an ebook for ‘free’ and go that route instead. There are quite a few JPod101 site rips out there as well as packs of Japanese ebooks, all ‘free’ for the taking. Now just think of all that lost revenue.
That said, TextFugu is due for an update and I highly doubt any changes will occur here as a result of the upcoming changes. If you wanted you could use an offline browser to save your webpages and then edit them as you like. You could also convert those pages to pdf if you so desired. Just be mindful of copyright laws in your country. ;)