How did past event like human trafficking (slave trade), prohibition, and Jim Crow show how society's beliefs and norms about law were in conflict?

In terms of acts of human cruelty in contradiction with American law, I think that it is rooted in America's challenges with the issue of human "difference."  We can see this in the earliest stages of American political theory.  As early as Jefferson's writings before the Constitution, we can see him extol the virtues of political freedom and self determination, yet falter and stumble through his discussion of slavery.  In fact, Jefferson was more "enlightened" than others when he suggests in his writings on the State of Virginia that slavery is something that should be banned, but must remain because of "inherent" differences between the races.  This contradiction between the practice of the law and the acceptance of its implications result in large part of social perception and the evolving social understanding about racial/ethnic and gender based levels of difference.  When American society has encountered something different, it has sought to demonize it or control it out of fear.  The American policy towards Native Americans highlight this and certainly its initial treatment of people of color has done the same.  Slavery was not seen as something horrific or excessively cruel until the narratives of slaves began to be heard.  For example, when Frederick Douglass begins to speak of his experiences, when Stowe writes Uncle Tom's Cabin, when "fugitive" slaves ran to the North, or when the Grimke sisters in the Carolina saw first hand the cruelty of slavery, we began to see social change evolve.  The expansion of American society's moral and ethical imagination is when the conflict between the law's theory and practice began to be bridged.  This imagination is expanded when we learn and understand more about that which we saw as "different."  Much the same process happened with regards to the Civil Rights Movement.  When we, as a nation, began to understand "separate, but equal" as actually translated to "separate and unequal", and when we saw protesters arrested for wanting to sit wherever in a bus or at a restaurant, social change began to happen.  When Dr. King writes "Letter from a Birmingham Jail" and when a nation sees the mutilated body of Emmett Till in a magazine, change happens to make our understanding of the law reflect its rightful practice.  If you wanted to look at it in a Constitutional way, it is when the Judicial Branch and the other two branches agree.  In this setting, our practice of laws becomes directly contingent on our social understanding.  Part of the reason that human trafficking in the modern setting has received so much attention is because we better understand the plight of those who endure it, causing us to apply our laws in accordance to our socially directed notions of the good.