Bullshit!
To Assert that the jury cannot try the law is to repudiate all jury nullification — and if that be the case then the jury cannot but find guilty all who are accused by the law if that law is sufficiently broad. Moreover, if a contraconstitutional law were to be passed (like, say, gun-control [the NFA or GCA or CT's new registration]) the jury could not regard the "law" as invalid but would be constrained to enact only the will of the state.
From Juries Are Allowed To Judge The Law, Not Just The Facts:
In order to guard citizens against the whims of the King, the right to a trial by jury was established by the Magna Carta in 1215, and it has become one of the most sacrosanct legal aspects of British and American societies. We tend to believe that the duty of a jury is solely to determine whether someone broke the law. In fact, it’s not unusual for judges to instruct juries that they are to judge only the facts in a case, while the judge will sit in judgment of the law itself. Nonsense.Moreover, Noah Webster's 1828 dictionary says:
Juries are the last line of defense against the power abuses of the authorities. They have the right to judge the law. Even if a defendant committed a crime, a jury can refuse to render a guilty verdict. Among the main reasons why this might happen, according to attorney Clay S. Conrad:When the defendant has already suffered enough, when it would be unfair or against the public interest for the defendant to be convicted, when the jury disagrees with the law itself, when the prosecution or the arresting authorities have gone “too far” in the single-minded quest to arrest and convict a particular defendant, when the punishments to be imposed are excessive or when the jury suspects that the charges have been brought for political reasons or to make an unfair example of the hapless defendant…
JU'RY,
noun [Latin juro, to swear.]
A number of freeholders, selected in the manner prescribed by law, empaneled and sworn to inquire into and try any matter of fact, and to declare the truth on the evidence given them in the case. Grand juries consist usually of twenty four freeholders at least, and are summoned to try matters alleged in indictments. Petty juries, consisting usually of twelve men, attend courts to try matters of fact in civil causes, and to decide both the law and the fact in criminal prosecutions. The decision of a petty jury is called a verdict.
In the ordinary course of jurisprudence, jurors should not try law.
That they have the authority to do that in the Common Law I do not dispute. Nor do I disapprove of it in cases where the law is repugnant to common sense or the consensus understanding of our Constitution [or constitution.]
That said, those are extraordinary circumstances, and in the routine practice of law, juries should not try law. That is what you are advocating, and it is a short road to legal chaos and all manner of criminal misbehavior by law enforcement and prosecutors.