The I-9 form is required to be filled out - or on record with the employer already - from all employees, citizens and non-citizens. Maybe they never asked the citizens to fill them out.
Why are citizens included? How else is it verified you are either a citizen or can otherwise legally work?
I’ve had dozens of jobs, all asked on the application if I was a citizen, I replied yes. They never asked for proof (citizenship papers). I think just answering “yes” as to U.S. citizenship may put the onus on the applicant, instead of the employer. Wonder if that would hold up in court?