The phenomenon of altruistic punishment itself is apparently not just a matter of speculation. Another quote from Preston's piece:

Experiments show that when some people punish others, the reward part of their brain lights up like a Christmas tree. It turns out we humans avidly engage in something anthropologists call “altruistic punishment.”

He links to this PNAS paper which uses a computer simulation to model the evolution of altruistic punishment. (I haven't looked at it in detail.)

Whatever the explanation for their behavior (and it really cries out for one), the anti-Knox people are truly disturbing, and their existence has taught me some very unpleasant but important lessons about Homo sapiens.

(EDIT: One of them, incidentally, is a mathematician who has written a book about the misuse of mathematics in trials -- one of whose chapters argues, in a highly misleading and even disingenuous manner, that the acquittal of Knox and Sollecito represents such an instance.)