Forgive me for butting into the CC forum, but the concern with "charging only" as opposed to "charging whilst using" is likely to be the risk of poor power management in cheap chargers when rechargeable cells become fully charged. (For all practical purposes it is wise for an airline to consider all chargers of consumer grade equipment to be "cheap".)
It is easy for damage to occur to rechargeable batteries that are over charged, and the results can be spectacular in terms of smoke and fume production, and the battery fires that result are not easily extinguished.
The likelihood that a battery will be overcharged is higher if the device it powers is not being used, so having this rule reduces the risk.
An aside - the core of the original post seems to be that rules (laws) are made to fit the lowest common denominator. My comment - most everyone thinks that they are above average. On average, about half of them are wrong.