Faulty product

bought 2 pendants with the same type of chain at the Westgate Mall store and my wife was really happy as it was a gift for the birth of our daughter. After a week she tried both on and the one immediately snapped when taking it out of its box (no tugging or barely any tension on the chain). took it back to the store to have it seen to, the staff member was helpful and i was told it would be checked for defects and will be repaired. when i collected it to my surprise nothing was done other than being told nicely it could not be repaired as its to thin and i need to purchase a thicker chain which would mean less money in my pocket and just filling up the coffers of theirs. my question, why sell a problematic/inferior product knowingly? this is not a reflection of the store staff (who by the way i was told that it happens often) but common business practise is a reflection on the entire company.