"if you're in a plane and you hit the ground, your head's proximity to your arse is the least of your concern."
True.
But if you're in a plane that experiences a disturbing but eminently survivable rapid deceleration event during take-off or landing, it would be a real PITA to find yourself severely injured or even killed by a blow to head from a rogue iPhone let slip from the hands of the prick a few rows behind you just for the sake of them not being able to put up with taking off/landing without just one more level of Candy frikkin Crush.
My guess: The Powers That Be have decided that keeping satisfying punters obsession with their fondlables outweighs passenger safety concerns on a simple actuarial basis.
i.e. the chances of anyone actually being hurt (or even killed) as a result of this madness are slim enough that the cost of paying out compensation to victims that are created (or their families) is economically viable.
Which won't alter the suckage for those unfortunate enough to find themselves a statistically acceptable victim.