Sometimes it is a cheap rhetorical device, but in my experience it tends to be due to the fact that most Americans have not spent much time deciding where they stand on whether rights are natural or socially constructed, which is a conclusion one needs to come to before having an opinion worth opining as regards legal rights current or proposed. In defense of my fellow Americans, that stuff is not all that difficult to muddle. And we do not really have an illustrious ancestor like Herr Nietzsche who has sketched out the difference beforehand to help guide us in our thinking.
Bookmarks