Greg - the shop people should not have been surprised. A lot of the dealers/distributors will install aftermarket wheels on trucks having given the suppliers truck specs. Sometimes, either through mis-identification of proper specs, too light duty a tire per truck or casting problems, cracks will spontaneously appear. There have been past NHTSA recalls for such issues. So while not common, its not unheard of.
Bottom line - glad that the dealer stepped up and made it right. Nissan dealers are not known for being on the straight and narrow (and I know this as a Nissan car owner!).