Consider this: A jury's decision is final no matter what, and you can't be tried twice for the same crime—that's double jeopardy—so a jury can find a defendant not guilty for any reason and he goes free. If either of those things weren't true, the jury could just not find a defendant not guilty as a symbol, but he'd still probably go to prison eventually. Since those two things are true, jury nullification gives a lot of power to juries.

This isn't just theoretical. Before the U.S. Civil War, runaway slaves were required to stand trial wherever they were apprehended. If the trial took place in a Northern state, juries would often demonstrate their outrage with Southern slave laws by finding the fugitive not guilty, even when all evidence showed he or she broke the law.

Not everyone thinks this is a good thing. For one, juries are often kept in the dark about many facts involved in a case, since factors like the suspect's criminal record and certain extraneous details are inadmissible in court but fully known by the prosecutor. And though it may seem so, jury nullification is not very democratic. After all, it was democracy that put the laws in place to begin with. This is one of the reasons courts aren't allowed to tell juries about the rule, since to do so might make for an unfair verdict.