I imagine you have never seen one then? The backlight on the paperwhite is actually a frontlight, so it doesn't strain the eyes, I find it a lot better than any old Kindle, or frankly - than reading an actual book.
Why should it matter for eye strain whether it is a frontlight or a backlight? In both cases, it is some amount of light reaching your eyes. What should matter would be just the brightness/contrast and uniformity of it.