There is no duty under the law to rat on fellow jurors. There is barely any such thing as improper reasoning. Jurors have the power to use any reason that they want to reach a verdict. Usually the judge tells them not to consider the punishment, but it seems neither possible nor desirable to completely ignore it. Under the minority reasoning, if a juror says, "I just don't think that he should go to jail for doing that", then the other 11 jurors have a duty to turn him into the judge to be kicked off the jury.
The minority says:
The majority also does not dispute that jurors have a duty to report such misconduct to the court. (See People v. Williams (2001) 25 Cal.4th 441, 451 [“jurors are required to follow the trial court’s instructions”]; id. at p. 452 [“ ‘the judge must be permitted to instruct the jury on the law and to insist that the jury follow his instructions’ ”].) The problem is that the majority, other than disapproving this instruction, fails to articulate how trial courts may properly inform jurors of that duty, apparently assuming instead that jurors will discover this duty on their own.
The simple explanation is that the duty does not exist. Judges should not be telling jurors about a duty that does not exist. The minority is wrong to think the duty is implied by an obligation to follow a judge's instructions.