Disclaimer: I'm not a lawyer, there are definitely people who can explain this better, and I'm probably using these terms incorrectly.
Yes, mens rea is an important consideration, but it's nuanced. Check the Model Penal Code [0], which identifies four differing types of mens rea, including negligence and recklessness; that is, a "guilty mind" (mens rea), for criminal purposes, does not necessarily require what would be conventionally considered bona fide malice or intent to harm.
What I meant when I said an "enhancement" or "aggravating factor" is that usually you have an objectively asocial actus reus, like theft, and should mens rea come into play, it's generally a defensive thing seeking to exculpate the accused, as in "I didn't know it belonged to someone else" (the affirmative defense), not to deny the act.
But with insider trading and other instances of nuanced malum prohibitum [1], the usual relationship between actus reus and mens rea is inverted. To make the crime, one starts with the mens rea, the bad intent, and must identify (or, if necessary, manufacture) an apparently-normal act to register as the external offensive conduct that harmed society and warrants legal action.
That is a scarier proposition because if your daily business involves technical acts that can be converted into actus reus, there's obviously going to be ample opportunity for people to assign and rationalize their preferred ideas about your thought process there and convince themselves that you're a criminal based on their personal level of dislike or offense. If this gets brought in court, your defense will amount to convincing the jury to believe you instead of the prosecutor, which is a straight-up likability and performance contest.
Whereas, with better-defined crimes, there is a physical, independent actus reus that people recognize as objectively bad and probably intentional. If you didn't steal the thing, if they can't show that you stole the thing, that's now the ground that you're fighting over, and that's much better for the defendant because it's much less fickle.
Essentially it makes every defense necessarily affirmative because the conduct is not otherwise unlawful. The government must dislike you enough to assume bad faith first.
