Actually the federal law does not make it a felony not to carry documentation all the time. Its not possible to carry documentation all the time. Its not just one document you know. For people with greencards, maybe but not others. There are plenty of ways you can prove that you are here legally without carrying 2 to 3 different pieces(Passport, I-797 or I-20, I-94) of document that are required to prove that you are legal. Lose those documents you have to wait 2-3 monts and spend $$$s trying to get duplicates. You cannot get driver's license in any state without proving to DMV that you are here illegally. Also, are the local LEOs going to check with federal databases if the paperwork that someone gives them is authentic or not? I understand why this law was enacted but its impractical to work the way its supposed to.
As a legal immigrant, I have spent years on F-1 and H-1B and more years just waiting on my greencard, I don't mind that I have to prove that I am here legally when needed but to ask me carry documentation all the time is at best debasing. Also, there are more people legally than illegally. What baffles me is why the businesses that employ illegals are dealt with stronger?
Will get off my soap box now.