With anti-trust laws, as with regulatory commissions, a sharp distinction must be made between their original rationales and what they actually do. The basic rationale for anti-trust laws is to prevent monopoly and other conditions which allow prices to rise above where they would be in a free and competitive marketplace. In practice, most of the famous anti-trust cases in the United States have involved some business that charged lower prices than its competitors. Often it has been complaints from these competitors which caused the government to act.
Thomas Sowell, Basic Economics (2010), Ch. 7. Big Business and Government