I've been meaning to ask about this. You don't really "turn off" a Kindle - there's only temporarily a charge when it's loading a new page. Just showing a page does not mean it is "on".
What is the consensus about ebooks? I tend to just close the cover when landing as a gesture of goodwill, but it doesn't actually change its electric activity compared to when I'm looking at it.
As to the OP's example, you can expect a stern talking to if you're ignoring the cabin crew's requests (which is against your terms of carriage and possibly illegal), but any crew using the term "taking the piss" when dealing with a passenger is likely both in need of an attitude improvement and some basic customer service training.