And aside form good-ole jury nullification, there's the simple truth that people have emotions and convictions. That's one of the reasons potential jurors get bounced during voir dire. Last time I got called, the fellow next to me told the prosecutor quite honestly that he didn't like cops and didn't feel he could trust one to be honest. That made him unqualified to sit on a DWI jury. Similarly, if I think laws prohibiting marijuana are bullshit, it's pretty likely that they're not going to want me on a jury. Now, they can certainly tell me to put my opinions aside and act solely on the evidence and the letter of the law, as they could have in Montana, it's the expectation of a jury, after all. That's a pretty shaky thing to rely on with a 12 member jury who are all opposed to the law, though. If it were a murder trial, that's probably what would have happened, but in this case there really wasn't any point.