They don't want people looking at their devices with their headphones in when the captain says "brace for impact" a moment before you're supposed to land normally. It's not that hard to just be ready for an important announcement before takeoff and landing. And they're right that you want everything stowed away for those two phases of the flight.
I'll take my chances that even if I did brace for impact it wouldn't make a significant difference in my survival or chance of injury. And whether I'm looking at my kindle, staring out the window, even staring right at the flight attendant in the jump seat, I don't think it's going to affect my reaction time at all. Even with headphones on I can hear cabin announcements (I sure wish I couldn't, so I could sleep while the captain points out that we're crossing over the Rocky Mountains).
I don't remember ever being asked to stow a book, and my kindle is smaller and lighter than most hardcover books (even many paperbacks). Besides, I've seen the overhead compartments come unlatched during severe turbulence, so in the event of a real crash, a loose kindle is the least of anyone's worries.