> the United States has spent a long time having a tacit policy of only partially enforcing its immigration laws

This seems relevant. In other policy areas (unrelated to immigration), there's the concept of gradual change, or a 'glide path'. For example, when raising prices on things like licences or levies, sometimes governments do it gradually over a number of years. Another example: when something is banned, the ban is not enforced immediately but intermediate steps are taken to ensure the public can adjust. By contrast, the transition from immigration rules being unenforced to enforced seems to have been sudden.