The argument of why Capitalism is a system of justice stands on the principles of individual freedom, individual rights, freedom from any type of coercion and equality of opportunity.
For a system to be moral it needs to allow its members to not be treated immorally. As such, capitalism allows for individuals to be able of doing whatever they please to do in order to be happy. In a capitalist society, its members are able of pursuing whatever they wish in as much it doesn’t violates other individual’s right to be free.
In order for individuals to be free they need to have some certainty or security of what those rights to be free actually are. Therefore, individuals need to know what their rights to be free depend upon. Those freedoms are established by common agreements and traditions first, mores and oral commitments and in more complex and civilized societies by a Constitution and legislations. This agreements on what an individual could do, need always to be consistent with their freedom to act, without anyone telling them what to do.
As such, in a capitalist society all the individuals will have equal rights to act upon and equal rights to demand for justice when their rights are violated.
In a capitalist society there will be rewards for the work a man does. He will receive a salary in return for the services and work he did. That salary is the worth of a person’s work in order to produce something. Some individuals will earn more and others less. Their work is not valued by the overall good he gives to society, but by the product of it.
But how can it be a moral system if the people that works harder (labor force) earns a lot less than the rich?
It may be true that a man that sells ice cream in very hot summer days provides an overall “societal” benefit that is higher to his boss’s. However, the boss of this person who earns only 7 dollars per hour may be earning a lot more, maybe 25 dollars per hour. This is just. Why? Because the boss had to work in a different field in order to achieve a higher salary. The product of his work is worth more to the owner of the ice cream company and he also had to comply with a different job profile when hired. He had to study in College and this boss is able of making the business grow and make more money for the owner of the company. Such as Manning’s example and all the money he makes for those companies that profit from the baseball, basketball, and other businesses.