You're violating federal labor law. Specifically, the Immigration and Nationality Act (INA), 8 U.S.C. § 1324b.
"Employers may not treat individuals differently based on citizenship or immigration status. U.S. citizens, recent permanent residents, temporary residents, asylees and refugees are protected from citizenship status discrimination."
Friendly intro: https://www.justice.gov/crt/types-discrimination
If anyone applies for this job and is denied based on citizenship status, here's where to file a charge: https://www.justice.gov/crt/filing-charge