This is a common misconception. American Express charge cards (Green, Gold, Platinum or Centurion) normally have no fixed limit, but this does not mean unlimited spending. American Express will not reject transactions automatically, but rather they will evaluate them based on the information they have about the cardholder's assets, income, payment history and spending pattern.

In this instance it appears American Express is experiencing lender's remorse, probably upon realizing that the cardholder is a student, and has elected to notify him of the limit.