Short answer is "They don't go that dim." I don't know if there's a technical limitation or it's just not done, but the "1%" frontlight (most e-ink displays ar

Short answer is "They don't go that dim." I don't know if there's a technical limitation or it's just not done, but the "1%" frontlight (most e-ink displays are frontlights, LCDs are just awful for every aspect of reading) is quite a bit brighter than reflected. E-ink readers absolutely can be read without backlight, and I've experimented with that vs the frontlight for the past few months. The older ones (my Kobo Aura) require a lot of light to be readable, but are readable without much trouble by kerosene lantern. The newer ones (Kobo Clara HD) are better, and require less light, but all of them require an awful lot more light than a paper book.