Mike, the simple answer is because you were told to. The instruction is probably to "switch off all electronic items." Airlines can't be expected to keep lists of every single piece of electronicery (updated daily) and judge whether it "transmits" or not, and then train the cabin crew to recognise them all. How could they?
As with so many things in life it comes down to keeping rules simple.
(In reality anything electronic transmits RF of some kind, so it cannot be inert unless it is off)
It's no more a book than an i pod is an orchestra, its a piece of electronics and falls squarely under the simple safety related request quoted above.
I do take your point on the safety demo though, if they were consistent on safety they'd enforce attention on that as much as switching off gizmos, but life ain't like that, though the much maligned Ryanair has a reputation for cc barking at backsliders during the demo, bless 'em.