It's a flavour of social darwinism. The argument essentially goes that you encourage darwinian evolution of society where the weak are indeed weeded out, and that we should encourage people to "evolve" and become "strong".

In my experience it's usually a rationalisation by those who wished to do the thing already, but now even more than apathy, there's a moral obligation to bully people, which is quite appealing if you want to do those things.