It's generally accepted that for reading on a screen, justified text is slower to read; hence the recommendation for websites is that left-justified text should always be used. It's also preferable for people with dyslexia, whose word recognition can be disrupted by the variations in spacing needed to make justified text. (See
this RNIB page for details).
However, whilst I'd agree with the view on normal, light-emitting screens, I find personally that on an e-ink screen there is no drawback to justified text; despite having slightly odd eyesight, I'm not aware of any more difficulty reading on e-ink than on paper, whether the text is justified or not. A ragged margin is mildly distracting, so I prefer justified text on paper and e-ink screens.
The BNIB identifies the refresh rate of luminous screens as a factor; if they are right, then the absence of refresh on an e-ink screen would explain the difference here. Personally I suspect it also has something to do with light being emitted from, as opposed to reflected by, the screen, and as a result a greater contrast for the eye to handle between the extent and the surrounding environment.