The INA also makes it illegal to recruit someone for employment if you know he/she is an illegal alien and prohibits hiring a person without verifying documentation showing that the person is authorized to work in the U.S. So the city of Plano, by creating a place where day laborers can find employment, is actually in violation of US law. There is no way the can't know that the majority of the people that cogregate at that location are illegal immigrants.
Maybe some concerned citizens will file a complaint against Plano with the INI.