Dusty, I don't know about the ID point, but as part of a trading faction, I think it make total sense for Xenos to demand cargo. The point is to disrupt foreign trade, and that seems to be the most direct - and symbolic - way of doing it.
That said, I wouldn't always demand all cargo, but enough to make the run expensive. Demanding all is going to simply prompt the trader to say "Are you kidding?" and refuse, returning everybody to point A without much RP.
Otherwise, for whatever it's worth, I think it's totally appropriate.