A recent Vanderbilt study estimated that a 10% cap would save borrowers $100 billion a year in interest costs—money that banks wouldn’t get. The study said rewards for cardholders with credit scores below 760 would likely be cut to make the math work.

Amid voter concern about the economy and affordability, President Donald Trump announced a new policy: a 10% cap on credit card interest rates for one year.

Synchrony, which issues cards for Amazon among others boasted a 25 per cent return on equity in its latest quarter, even factoring in allowances for credit losses equivalent to around 10 per cent of i...