Q : What authority do you have to collect my information?

Congress mandated that USCIS provide a system that verifies whether a newly hired employee is authorized to work in the United States in various statutes as provided below:
 
Immigration Reform and Control Act of 1986 (IRCA), P.L. 99-603, dated November 6, 1986:
IRCA required the creation of a system for verifying the immigration status of non-citizen applicants for employment.

The Illegal Immigration Reform and Immigrant Responsibility Act of 1996 (IIRIRA), P.L. 104-208, dated September 30, 1996:
The IIRIRA required a means to respond to inquiries by federal, state, and local benefit issuing agencies and institutions seeking to verify or determine the citizenship or immigration status of any individual within the jurisdiction of the Agency for any lawful purpose. Title IV of the Act requires the establishment of a Basic Pilot Program with voluntary participation by Employers who could use this system to determine whether newly hired employees were authorized to work in the United States.