The problem with Kindles and their competitors, is that they have no light installed in them. At least the original ones.
So you need an external light source to read them.
Where I work we get bussed to and from site which can be a 30 to 90 minute ride. There is no lighting on the bus for reading, so you have to bring your own. Many people bought Kindles and then realized this little "glitch" after.
Most people buy Kindles and assume they have a built in light because they are electronic, but they don't.