Actually the issue is not primarily based on radiation or stuff like that (althogh tere were severe probs with cellphones 10 years ago), but on
1. Heavy devices not secured during takeoff and landing (Laptops, Netbooks etc.)
2. Divided attention if using a phone / MP3 player or stuff like that
Beginning and end of flight are commonly agreed the most critical during a flight, and in case of an emergency it is really necessary to understand commands issued by the flight attendants or pilots.
As for phones during cruise flight: they won't work anyway.
Where do Ebooks range on this scale? : Actually nowhere, but Airlines have a strong momentum in changing regulations, and you can't expect the staff to make differences in electronic devices (is it a reader or tablet, with WiFi / 3G, switched on / off), so they make it simple and forbid everything.
Not ok, but it's the law. Like DRM
