Like most of his fellow lawyers, Dershowitz wants to use our jury system but does not want it to exercise its right of nullification of the law.
Historically, the jury was instituted to protect the individual from the injustices of the king's law (oppression by government). The jury must protect a person from unjust laws as well as unjust accusations. Just as juries saw the Volstead Act as an unjust law and refused to convict many bootleggers, the Goetz jury may have seen the statutes limiting the justification to protect one's own life as unjust in this case.
Goetz's decision was "unlike the decision of the punks who surrounded him." Evidently, their intent was to commit serious crimes and Goetz's intent was to protect himself.
Dershowitz's complaint that the "jurors disregarded the judge's instructions" is without merit. It was their right to do so. The jury cannot be "required" by the judge to convict.