There is more to capitalism than just social darwinism. The sytem of capitalism controls how people effect their government. In American society for example, it would be safe to see the government as a large insurance company. To have that insurance company privatized (fitting a capitalist model) or socialized is something that is not black in white. Social security, healthcare etc all have different effects and not all fit the schema of capitalism.
Generally for a government/society to have a maximum effect on reaching it's goals (from a to-down approach) it has to employ a method that works to insure livelihood to it's majority. The obvious flaw is that the majority is situationally favored - this is why many traditional models of economics (like the invisible hand mentioned by Adam Smith) are WRONG. Information is often asymmetric and to say that some ideology is universal is very poetic but not accurate.