The "jury of your peers" exists to provide a contextual moral component to the law. If your peers think you did no wrong, it doesn't matter what the letter of the law says: a crime that is justifiable to society deserves no punishment.
Conversely, if what you've done is not justifiable to society, you should be punished for it even if it is not a crime. The easiest way to understand this is when first-mover advantages run amok. If you're the first person to think of some dastardly evil, you should not be permitted to do it just because nobody has yet written a law prohibiting it. Like, say, the Coca-Cola Corporation is not innocent for putting addictive substances in their drinks simply because it wasn't illegal at the time. And Shkreli's price gouging doesn't sit well with anybody either. A person's ignorance about some harm does not constitute permission to commit that harm against them.
>When has the average person been able to grasp complex matters relating to say, securities fraud?
The defendant has an attorney whose job it is to explain these matters to the jury.