So, the problem is that the term "parasite" has not been scientifically declared in social context? Then I think what we have here is an analogy, rather than a metaphor.
As for why, here's my reasoning.
A. Two groups of people, group "A" and group "B" (both groups sized 1...n)
B. Group "A" uses the material or mental resources of those in group "B", without consent of group "B"
C. In the process, group "A" gains, group "B" wastes away
And because of point C, I think this analogy CAN be determined in social context without arbitrariness and without taking cultural factors in consideration.
All you need to do is record the start and end states of both groups, and the flow direction of "goods" between the groups.