Devil's advocate here. If Carmen actually agreed that Aaron's actions were not a crime, but wanted most of all to set a precedent, could that not explain her actions without malice? Maybe she was ideologically on Aaron's side, but recognized that a weak or cancelled prosecution would not yield a useful precedent. If she demonstrated a strong case and lost, it would send a message that Americans don't agree with strict enforcement of copyright in that way.
Of course this would be little solace to Aaron, as he probably would not be made aware until afterward, if ever.
I suspect that having a lost prosecution on her record would be more harmful to her own goals than helpful to the goals of any causes, so this explanation seems unlikely to me.
That falls to Occam's Razor. Prosecution is the way to conviction, so just publicly choosing not to prosecute would be a strong message.=, ad much lower cost.
Also, federal cases usually get settled in pleas, so there's less actual precedent. Going all the way to court would have been painful for him AND would have increased the odds of conviction.
Of course this would be little solace to Aaron, as he probably would not be made aware until afterward, if ever.