The U.S. Supreme Court missed an opportunity to do something about income inequality. The opening came in the form of Ohio v. American Express, the state joining 16 others in arguing that the credit-card company violates antitrust laws through its contracts with merchants. In siding with the company, the 5-4 majority not only overlooked how the arrangement diminishes competition. It settled for a payment system that benefits wealthier consumers at the expense of others.

That payment system is notoriously opaque. If customers readily can find in their bills the interest charged or the annual fee, other costs are practically hidden. Consider the “swipe fee” that credit card companies charge merchants. Visa and Mastercard apply fees ranging from 2 percent to 3.5 percent of the cost of a transaction. American Express puts its price between 3 percent and 5 percent.

The difference goes, in part, to the way they do business. American Express relies heavily on its more attractive rewards program. In contracts with merchants, the company also prohibits the business from informing customers that there are cheaper alternatives than American Express.

The question before the court was whether this “anti-steering” language harms competition. Should a merchant have room to charge customers different prices depending on the credit card they use? The majority broadly answered no, finding, among other things, that sufficient competition exists in the battle of the rewards programs. It noted, too, the credit card market has been expanding.

What, then, is the problem?

American Express customers enjoy a subsidy. Their card costs the merchant more, yet they pay the same prices as those using Visa, Mastercard or cash. In addition, the anti-steering language means that merchants are barred from telling customers: If you paid another way, the business would be in a position to reduce its prices.

It’s no secret those with lower incomes are more likely to use cash, prepaid cards or debit cards for purchases.

Why don’t merchants just abandon American Express? Many like the customers, who tend to be wealthier and spend more, collecting those reward points. Here, income inequality makes a notable appearance. Rewards programs translate to money back, in the form of such things as airline flights, hotel rooms, even cash.

As Aaron Klein of the Brookings Institution explained in a post this week, a family charging $80,000 a year on credit cards could see as much as $1,200 (tax free) as a result of a rewards program — an avenue out of reach for most households.

It seems the least customers are owed is transparency in pricing, merchants with the go-ahead to say: There is a cheaper way. Without such an opening, the credit card payment system aggravates income inequality. American Express keeps its protection from price competition, something contrary to antitrust law and what the court majority failed to see.