I do not believe you understood my question.
How is it a step backwards to ask people for identification. This happens to everyone constantly. To single out a segment of society and exempt them from this process is itself discriminatory.
No one is asking people to prove that they are an American, but that are legally in the country. We assume this of citizens and require it by law for non-citizens.
Why is it bad to ask people to comply with the law?
Completely outside the purview of the original question how is it bad to enforce immigration law?