It assumes that man is greedy. The capitalist does not want man to be greedy, but recognizes, as an irrefutable fact of life, that man is greedy. That even those who criticize greed are themselves greedy. In light of this assumption, capitalism understands that a greed motivation is necessary to get one man to work for the welfare of others. Greed is not a recent phenomenon, and is not going to be eliminated from the government by electing better people, who like all of us, possess a greedy nature.
Capitalism also assumes the profit motive serves a cleansing function.
Bad business ventures go out of business when they loose money, because greedy men don't like to loose money. In contrast, poorly conceived projects of government usually continue, on and on and on, because the people who benefit from the programs, both bureaucrats and recipients, who are also greedy, will fight to keep everyone else involuntarily paying the bills. To be fair, it should be pointed out that many businessmen, motivated by greed, ask the government to give them advantages over their competition. A true capitalist should condemn such requests. Antitrust legislation is necessary to restrain the greed of businessmen and maintain a level playing field.