^^ Some states, such as South Carolina, passed laws aimed at keeping "cheap" guns out of the hands of those people they felt would be most likely to use them, which they believed would only be thugs and gangers. Since price points change, they went after material of construction. The laws required the metals be of a quality that would withstand exposure to a certain level of heat without melting, knowing that the zinc and aluminum alloys used in the guns they were trying to ban would not pass.
I'm not sure how many of these laws, however, pre-date the arrival of polymer-framed guns, or why they continue to be exempt (well, actually, I do know, but that's for another discussion.)