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Hmmm, the easiest way I know to explain this is through an analogy. I hope you know what an "Etch-a-Sketch" is?
There are two ways to provide power to an etch-a-sketch (EAS). 1) You can turn the knobs and draw on the screen. 2) You can shake the EAS and erase the screen. If you don't provide any power the screen stays fixed on its last image. Apply power by either turning the knobs or shaking it and the screen changes.
Just like the EAS, once a 'picture' is on an e-ink display it stays there unless power is added. Even if you want the picture to disappear you need to add power in order to make it disappear.
When you hit the power-off switch or the reset button, the Bookeen engineers had to decide how to turn off the book reader. For example, they could leave the screen unchanged and simply turn off the power. But if the screen doesn't change how do you know if the button press worked? Is the book really off, or did you fail to hit the button correctly?
The Bookeen engineers chose to clear the screen to tell you that the system is off. This is a sign to you that you hit the button correctly. However, it actually takes power to clear the screen before the system shuts down.
So the blank screen is not a sign of the power going off. It is simply a user interface trick that is used to tell you that you hit the off button correctly.
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