Charged for cancellation despite being unable to offer service

My son recently had issues on his MTN contract. The account was up to date but he was unable to make calls. Various attempts to get it sorted with MTN resulted in no positive outcome. We then decided to take advantage of a Black Friday special with another Network and requested a cancellation with MTN. They charged us more than R1600 for cancellation. How is it that a company is unable to provide a service or resolve an issue and still be able to charge their clients for cancelling their contracts as a result of the above?