If they are legal and not breaking any laws, they have nothing to worry about. It is common sense.
I understand where you're coming from on this but I've never been able to accept this line of reasoning.
One could just as easily claim the 4th amendment ugarantees against the police entering your home to look for illegal drugs is just as dispensible so long as you do not have illegal drugs in your home.
Now, legal aliens are here by permission and a requirement of that permission is that they maintain on their persons such documents as may prove they retain that permission. It's akin to you having permission to dine in my home so long as you do not drink my last beer (not that I ever want to be found in the position of having a last beer).
But what about US citizens?
Well, when they are stopped for a traffic violation--a thing requiring probable cause--the law has a right to inspect for proper licensing but apart from the probable cause of an underlying offense this law properly protects the rights of me, the US citizen, who may be mistaken for a foreign rabbit...or worse: a ptarmigan.
I think the closest to thing to an immigration dragnet would be a DUI checkpoint.
It would be nice to see the law expanded to include enrollment in public services such as financial assistance and schools.