Arguing with low wage employees while they're having to deal with hundreds of passengers during the short pre-flight legally mandated checks makes you appear like an arsehole. They don't know every device on the market, nor whether that device is using 3G or wifi or is just sleeping or is off. And they don't need to know, they've been told to make sure that everything is turned off and that there are serious consequences if it doesn't happen.
The fact that the regulations are idiotic is not the fault of the busy, underpaid, low-skilled, attendants.
[1] https://kindle.s3.amazonaws.com/Kindle_User%27s_Guide_Englis...
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> Power switch — puts your Kindle to sleep, wakes it up, and turns your Kindle on or off. The power switch is located just to the right of the micro-USB/power port. To put your Kindle to sleep, slide and release the power switch; the screensaver appears on the display. While your Kindle is asleep, other keys and buttons are locked so that you don’t accidentally change the place in your reading. For Kindle with Special Offers, the center of the 5-way controller is not locked while your Kindle is asleep. To turn your Kindle off, slide and hold the power switch for seven seconds until the screen goes blank and then release.