Short answer, the Constitution limits the power of the government.
Her family, legally that is her husband, made a decision, (Glad I'm not in his shoes.), and because we are a nation of laws, we have to live within them, even if it seems to stink sometimes.
Sure, sounds easy. But the person recognizes that what the State is doing is Killing one of it's innocent citizens. They recognized it as State sponsored murder. Which it is. And that doesn't go over real big in Iran.