I'm not an expert, but there was some kind of legal requirement for the spent cases in some states. Because a manufacturer had no way of knowing where a particular gun might get shipped, they just included the fired cases in everything. The law went away when surprisingly enough it was finally realized that not one crime had ever been solved using all those probably millions of spent cases stored away in the Indiana Jones warehouse.
That's probably not 100% accurate, but it's more or less the gist of it.