Why do we have antitrust laws?

The Supreme Court has called them "the Magna Carta of free enterprise", United States v. Topco Assocs., Inc., 405 U.S. 596, 610 (1972), and the Sherman Act "a comprehensive charter of economic liberty", Northern Pac. R.R. Co. v. United States, 356 U.S. 1, 4 (1958).

Sounds great. But

The attorneys-general of eight states plus the Antitrust Division of the U.S. Department of Justice have settled antitrust claims against Visa and MasterCard over their restrictions on merchants' freedom to steer customers away from cards that carry high acceptance charges. 

They also sued American Express after it declined to settle.

The pact with Visa and MasterCard