Why, other than the courts saying so, shouldn't juries judge the law? Even in states that allow juries to do that I don't see anarchy erupting, but I do see a lot less of prosecutors getting away with throwing people in jail for stealing candy bars. Do you honestly think the 3 strikes law would be tossed out if it was used against a serious crime? Or that it should be applied to someone who is hungry? Wouldn't you love to be able to argue to a jury that the law, as it is sometimes applied, is wrong, but that it works more often than not?
A major problem with your argument here is that you are limiting it to the cases of obviously objectionable laws, such as marijuana possession laws and third strike laws applied in the case of a minor, triggering offense. But if "jury nullification" had official sanction, it would mean that any juror would be free to throw a monkey wrench into any criminal trial, merely because he or she "didn't like" the particular law involved.
Juror No. 7 thinks the drunk driving laws are unfair because he feels he was unjustly convicted of DUI a number of years back. He knows the trial he is about to sit on will be a drunk driving case, so he fails to disclose his history on voir dire, just so he can "get back at" The System, by "nullifiying" the law in this particular case.
It doesn't take a genius to see why jury nullification is against the law. And while we're at it, was it you who was claiming that jury nullification is actually
provided for in the constitutions of several states? Link, please.