The display taechnology used in the Kindle (and other eBooks) does not require power to hold an image. It does, however, require power to change the image.
So, strictly speaking, in a true "off" mode you could continue to read the same page that was displayed when you "turned it off", but you could not "turn the page".
However, just as noise cancelling headphones are now allowed during TO in some cases, I expect that regulators will eventually catch up and allow Ebooks.