It's so basic, I'm amazed that people still wonder whether "iPads make the navs go all crazy." Has nothing to do with it.
What a load of cr@p and shows how little you understand the primary motivation.
An airline, the manufacture and the regulator need to ensure that any equipment used during critical phases of flight has been appropriately tested to ensure that it does not interfere with any systems.
Airlines in any regulatory regime of any repute will undergo some form of testing (or rely on others' testing) to ensure that two/three iPads in the cockpit won't interfere with the aircraft. In some cases (but not all) this testing is done specific to an ipad's serial number.
This testing does not involve 230 people tapping away on 230 iPads in the landing phase. Until that happens passengers are obligated to turn them off.
Until the regulator/manufacturer/operator test a hundred or so ipads down the back you may not conclude that it is ok to keep them on.
There is a secondary benefit in having attentive pax, but that is well secondary.