The judge can instruct him as to the pertinent law in the case. But court cases aren't based just on the applicable laws, but also on the facts of the case. The jury determines the facts; and then decides whether or not to impose a penalty. If a juror decides the application of the law would be "unfair" for some reason — even if the evidence points to the defendant's guilt "beyond a reasonable doubt" — the juror can refuse to return a guilty verdict. In effect, a jury nullification says, "for reasons that satisfy me, the juror, I will not apply the law to this particular defendant."
Federal and state authorities have the power to indict (upon the direction of a grand jury), but they do not have the power to convict. Only a "jury of one's peers" can convict. In many if not most cases, it requires 100% unanimity among jurors to convict. The state cannot punish unless the jury decides the defendant is guilty.
The OJ case is a prime example of jury nullification. Most people detest what the jury did in that case. But they were well within their sovereign rights to find as they did. It's been suggested they nullified to make a clear statement about the misconduct of police and prosecutors in the case. Evidently they thought that was worse than what OJ was "alleged" to have done. Whatever. In any case, OJ walked.
Jury nullification could be an excellent tool in RKBA cases. It usually only takes one juror to prevent a finding of guilt.
However, a jury nullification only applies to the specific defendant in the specific case. It does not and cannot establish a legal precedent for other like cases.
But it makes it a lot harder for DAs and courts to successfully railroad people for exercising their constitutional rights.
Bear that in mind if you're ever called to jury duty! It's one of the few opportunities in life nowadays when the individual citizen truly can be what the Constitution says he is: Sovereign.
Jury nullification gives judges the willies. Therefore, at voir dire — the interview that determines whether a potential juror will serve or be dismissed — it's best not to say anything about your knowledge of the law in general, and jury nullification in particular. (Unless you want to be dismissed speedily from service.)