I'm sure that the institution of slavery contributed to this, but the social Darwinism you describe seems quite consistent with the do-it-yourself frontier mentality. These "libertarians" view themselves effectively as antiheroes in the Old West: no free lunches, and everyone's got to carry their weight. Surely all the ways in which America was founded contributed to that ethos, and that's the main difference between American democracy and the social democracies of Europe.