WHY MUST WE PAY INTEREST ON A 6 MONTH NO INTEREST

A watch was purchased fr this branch during December. Upon reviewing the account statement it was picked up that a payment for January was not debited. After going back and forth to the store and bank including numerous calls to the call centre , it was discovered that a wrong account number was recorded while the details were processed for account approval. NOW WE ARE LEFT PAYING INTEREST ON THIS ACCOUNT BECAUSE OF ONE ERROR THAT WAS NOT OUR FAULT. PAYING INTEREST ON AN INTEREST FREE ACCOUNT ???!!! THE CALL CENT ER STAFF IS RUDE AND UNHELPFUL UTTERLY DISAPPOINTED IN THE AMERICAN SWISS CALL CENTRE STAFF!!!