In the opinion of a court, which would rather not recognize the concept at all. Courts hold a dim opinion of jury nullification, going so far as to legally sanction mentions of it in a courtroom, and to provide "instructions" to the jury that try to expressly prohibit them from considering anything other than whether the facts of the case meet the law as explained to them. Even hinting that you might understand the concept will get you thrown off a jury. In some cases, bringing up the concept during jury deliberations will result in a mistrial.
Personally, I like the way the state of Oregon handles the issue. Quoting the Oregon constitution, article I, section 16: "In all criminal cases whatever, the jury shall have the right to determine the law, and the facts under the direction of the Court as to the law, and the right of new trial". That said, I don't know whether in practice Oregon courts do any better about not censuring jury nullification.
It's a mistrial because you're telling the judge he's not important. Of course judges don't like that, they worked their whole life to be a judge, and some jamoke without a law degree shows up talking about jury nullification. That's why you see censures and mistrials.
Personally, I like the way the state of Oregon handles the issue. Quoting the Oregon constitution, article I, section 16: "In all criminal cases whatever, the jury shall have the right to determine the law, and the facts under the direction of the Court as to the law, and the right of new trial". That said, I don't know whether in practice Oregon courts do any better about not censuring jury nullification.