It makes it against the law for an "illegal" to solicate or take a day labor job. It makes it illegal to pick up any day labor from from the street. There are "day labor" hiring spots in Phoenix metro area where day laborers and people who want to hire them can go.
If you are an American, or legally in American you can work as a day labor. You just can't camp out at Home Depot to try and get a job.
Yes it makes it against the law to hire or be hired if you are in the USA illegally. Whats the problem with this?