Back when credit cards were still charge cards too. When revolving credit cards (where you don't have to pay each money) changed their names to credit cards (credit is a positive, charge is a negative), AMEX kept the charge card description.
AMEX reminded me of this twice when I didn't pay in full at the end of the month. They said they didn't work that way and I wouldn't be able to charge anything until I paid it off. I replied that I didn't mind because I only used it on corporate trips and I didn't have another one for months, by which point the company would have reimbursed me and I would pay AMEX.