The reason that "they" don't like jurors who claim knowledge of the "Law" is that it is not the job of a Juror to know it. The job of the Juror is merely to evaluate the evidence and testimony that is presented to them in the trial and follow the instructions as presented to them by the Judge.
That's the system, like it or not. Based on observations of recent trials in foreign countries as reported in our news, we have one of the best systems in the world.
How'd you like to be charged with a crime in Italy, sit in jail for years before a trial, and then be charged again for Slander because you called the police abusive?