Gas kettle enamel breaks off and kettle rusts

Bought one of thier very expensive gas kettles a year ago. The interior enamel came off at different places . When I took it back as it was under guarantee, and assistant told me it was 'water damage' as I left the water in the kettle after using it and therefore it was my fault. I told her that nobody told me to empty it each time and dry it. Herm**** assistant not interested.