Rhetoric: Amulek quite strongly states that those who disobey the king’s communal rules will be cast out, as dross. These rules are not optional but rather firm requirements. Failure to obey means exclusion, which, as any ancient people would know, means expulsion from the king’s protection.
Culture: While the imagery of refining ore (“dross”) is also related to Joseph Smith’s biblical familiarity, Nephite society had a record of metallurgy under Nephi (see commentary accompanying 2 Nephi 5:11); assuming that this technology had continued, they might be familiar with the image independently.