The Justice Department sued American Express Co., Visa Inc. and MasterCard Inc., accusing them of violating antitrust laws and citing rules that prevented merchants from encouraging consumers to use cheaper credit cards. Simultaneously, the Justice Department settled with Visa and MasterCard, which agreed to allow merchants to offer discounts to consumers who use less-expensive types of credit or debit cards. The companies said the settlement, subject to court approval, did not involve any payment.