Isn't the question whether "Social Darwinism" is descriptive or prescriptive? It's one thing to say that Europeans rule North America because they were "fitter" in the competition for the land than those who were there first, and another to say that BECAUSE Europeans COULD conquer North America, it was their moral right and thus their "manifest destiny" to do so.

For me, Social Darwinism explains, whether or not it justifies. No, it isn't natural selection in Darwin's technical sense, but Social Darwinism is basically a metaphoric proxy for natural selection, the tendency of the best competitor, however it comes into being, to win in the competition for scarce resources. It's a useful idea, whether or not it is a good thing.