My hunch is that the original complaint is that he got stuck sitting next to a rude, entitled passenger who made it abundantly clear that etiquette was for suckers (butting to the front of the line, hogging bin space and apparently hogging the power outlet). Part of the issue stems from the fact that there are typically 2 outlets for 3 seats.

Anyway, my read on the situation is it's first-come, first-served where outlets are concerned, but that proper etiquette also requires that you free up the outlet when it's no longer needed, especially if you observe that your neighbor needs it. There's no etiquette violation for plugging a single device into the outlet rather than using a multiple adapter.
The only real hard and fast etiquette rule I know of where outlets are concerned is that it's rude to use ALL the outlets.

One other point on the IFE USB ports: in addition to (valid) security concerns, those are low power ports. They'll recharge an average phone at a relatively slow rate, but they're too underpowered for tablets and large-format phones/phablets.