Why do conservatives reject Darwinism in clrooms but embrace economic Darwinism?

Why do we have so many conservatives that reject "survival of the fittest" evolution theory, and don't want it taught in clrooms, or at least not exclusively (because, after all, the Supreme Court has repeatedly struck down Creationism in clrooms---another reason why conservatives like McCain-Palin want to overhaul the judiciary), and yet they so totally endorse the ideal of a free market where the strong survive and the weak subsist on what trickles down? Where ideally there should be no federal government at all except to raise taxes to pay for the military but little else, because anything else is tampering with the economic natural selection?