It's rather damning to their current view that they didn't need legislation. Clearly the lawyers at the Justice Department, or some of them, felt that legislation was needed to allow the government to do what it was doing.
It expands the jurisdiction that the Secret Service has over its ability to put in place these exclusion zones.
The provisions that are a problem are those that minimize judicial scrutiny of searches and surveillance and that increase government secrecy.
That it failed to effectively use.
But they do have the power to enforce federal criminal laws. So if you make unlawful presence a crime, they'll be able to enforce civil immigration law. They could charge immigrants or just transfer them to DHS, who could deport them.