My Grandfather, who was the US Attorney for west Memphis, once explained that prosecuting the law was messy. It was messy because there was rarely a case which hinged on exactly one part of the law or that had circumstances that nicely isolated the principle at stake. He was talking about pornography, and the difficulty of a general consensus that pornography was "bad" and should be outlawed, with the challenge of defining exactly what it was and the actual "bad" part of it.
I see a lot of parallels between his struggle to enforce pornography laws with the current struggle to enforce copyright laws. These situations seem to arise when there is a fundamental disconnect between what "the people" think and what "the system" thinks. "The system" is represented by a codified set of strictures that are put in place by a variety of people representing what they assert are the best interests of "the people." Whereas the people themselves, act in what they consider a rational way given their understanding of or perhaps agreement to, the laws of the land. Finally our system of laws are a combination of written text, and argued cases, and the sum of those is an emergent thing thought of as public policy. When the rational acting people don't consent to the public policy, there is a rash of disobedience, and whether it is alcohol, porn, or copyright, the process of emerging to a consensus is challenging at best.
One possible explanation for the zeal in which this case was pursued may be the lack of confounding factors with respect to copyright infringement, as codified by law. I don't know of course so this is just speculation. Having a clear, published, decision on the legitimacy or illegitimacy of what Aaron was doing might have been seen as a way to clear up a confusing pile of statutes and other decisions. An unambiguous marker between fair use and infringement, or perhaps a litmus test for intent. We'll probably never know.