Javonnie McCoy was growing marijuana when the cops came to his Middle Georgia home. He was caught red-handed with it. Almost a pound of it, in fact. He admitted it to police, and later he looked jurors in the eye and said, yep, it was mine. I used it as medicine. The jurors let him go. He was minding his own business and wasn’t hurting anybody, they reasoned. He just doesn’t belong in prison. The jury’s decision earlier this month in Dublin, Ga., may have been due to a muddled prosecution of a muddy case. Or it may have been jury nullification, another case of citizens saying prosecutions...