I'm really curious now: Why should you judge the underlying principle of those two differently? How IS the underlying principle different?
I currently can't see an obvious explanation except basically tautological ones.
Anything social is arbitrarily defined based on time (era), culture, and the size and history of the group defining it. Much as sociologists try to make sociology a science, it's a psuedo-science at best (and I say this as someone with a serious interest in the field) because there is always going to be an element of the subjective in any definition or term.
Biology, on the other hand, has definitions based on objectively observed fact. There is a set of criteria which makes up the fundamental basis of the definition which have to be objectively met - it isn't subject to interpretation (as I already pointed out to TELSA in discussing the parasitic nature of a fetus) no matter how much you want to make it so.
So, with that regard, a social parasite requires a judgment call on the part of the person making the declaration. A biological parasite does not. The two are not comparable. Though, it must be said, the harm which can result from either is not pre-determined. There are many people that can be called social parasites (depending on one's perspective) that could be considered to do a greater amount of harm than any single biological parasite.