But they are ok with the 2 lb book I'm carrying or the can of coke (or bottle of wine/whiskey) that the flight attendant just sold me knocking them in the head or the 300 lb beverage cart ending up in their lap? Airlines still let infants travel on their parent's lap, surely if they were worried about my Kindle hitting someone in the head, they'd be worried about the child becoming a projectile.
Sure, I understand that loose items are dangerous in a crash,but if that's a significant risk, the solution to that is to ban *all* of the items, not just the ones that happen to have a battery.
Beverage carts are stowed during takeoff, landing and turbulence.
Babies on laps are required to be secured by a special seatbelt during those times as well (including during turbulence).
And yes, I think that all objects with substantial mass should be stowed if there's any actual risk - not just your kindle but certainly including it.