Polygamy =/= Polyamoury. Polygamy involves one person (almost always a man) having multiple spouses, often economically and legally dependent on him, which tends to result in abuse (quite surprisingly), as well as a surplus of people who remain single, which has its own set of problems. You probably have a good idea of what polyamoury is. Because historically we've mostly seen the former (in the case of the US, with the Mormons and its branching sects), but not the later, laws have been written to deal with the issues provoked by the former, but admittedly it's about time to allow polyamoury to have its own legal framing.