Walt Serafin, a manager for a biotechnology company in Kansas City, Kan., is getting an iPhone as a reward from American Express Co.

That’s not what U.S. card issuers and their lobbyists predicted could happen when they opposed legislation limiting abrupt contract changes such as interest rate increases. The American Bankers Association, a Washington-based trade group, said the law would penalize cardholders with high credit scores, making them subsidize losses from lower-scoring consumers.