Rhetorical: If one does not obey the rules of the community, one should not be in the community. Amulek is suggesting that obedience to these communal rules not only is requisite of those who are of the community of the king, but that if they do not obey the rules of the community they will be cast out, as dross is cast out. Thus Amulek places very firm requirements on this community of the new king. Failure to obey means exclusion from the community. Any ancient people would know that expulsion from the community is expulsion from the protection of the king.
Cultural: While it is possible that the refining imagery is also related to Joseph’s Biblical references, we should also remember that the Nephtie society has been dealing with metals since the days of Nephi, so this would be an imagery with which they should have been amply familiar.