No easy answer
Throughout history, there are few instances in which the country followed strictly its own immigration laws. Congress and industry frequently sought exceptions due to changing economic winds. This created a disorganized and disjointed immigration system that is more often used as a way to drum up votes than an instrument of policy. (Not our words either.)
http://www.nctimes.com/articles/2006/09/28/perspective/16_04_149_23_06.txt
http://www.nctimes.com/articles/2006/09/28/perspective/16_04_149_23_06.txt