Subaru Impreza 2011: Seized wheel nut

In March this year we had a situation with a seized wheel nut, just after we had our 2011 Subaru Impreza serviced. The Subaru Service Centre replaced the nut and wheel stud free of charge, but it happened again last month when we had it serviced at the same place. This time we were charged for the nut and wheel stud, and when we disputed the charge, all they would offer us was a small discount and told us it was not covered under warranty. We have made a complaint in the post service survey, but Subaru have not changed their decision. What is the point of a new car warranty if the service centre can pick and choose what it decides to cover?