As a non-belligerent, non-plunderer in this game I can say that I am grateful to Inno for keeping harbors and Promethium safe from pilferers. The cost of goods in the Arctic Future Era of FoE is quite expensive and the number of goods required to produce Promethium is even higher. At my current level of play in the AF I estimate that each time I send my ship out on a (roughly) 24-hour voyage for Promethium I use 125 goods (10 of each AF good for ship repair and 75 for the crew) to collect between 75 and 85 Promethium. In addition, it costs 5 of each AF good to "promote" my crew members roughly once a week.
To allow plundering of Promethium from harbors would be the equivalent of having a goods building plundered for more than 125 goods a day. I am fairly certain that those who promote the plundering of harbors woud balk at the idea of having their goods buildings plundered at this rate. Inno was smart to prevent this from happening.