Slightly surprised anyone needs either a power pack (or charger) today as USB ports are so readily available.
Power packs are banned from checked bags by many airlines and extending this ban to include usage on board would seem to make sense?
Some airlines include in their safety briefing cards that if a phone or battery pack is lost down into seating to call cabin staff and for passengers not to attempt to retrieve on their own.
I can well imagine that an otherwise safe battery pack might become problematic if munched in a seat mechanism or otherwise damaged by heavy-handed retrieval.
There is no intrinsic reason for a small battery pack to be anymore troublesome than a phone.
The exceptions being that some of these the battery packs may not be subject to the same design restrictions and scrutiny to which cell phones are subjected.