In 1996 Congress recognized that immigrants should be able to financially provide for themselves or rely on their sponsors.  An affidavit of Support was required for certain immigrants to enter the United States.  This document has since proven to be a sham.  It is too costly for States to sue the sponsor to get a judgement in the amount of public benefits that were rendered to the immigrant.  Also even if a judgement is rendered, the sponsor can file for bankruptcy and erase the judgement.  Please see the link below for further law explanation.