Asking for this information is not unusual, but asking for it before having any significant contact with an applicant is highly unusual.
Employers have a right to know about an applicant's legal right to work in the United States, as well as a right to verify previous work experience. However, it makes more sense for an employer to advise the applicant at the outset that an offer of employment will be contingent upon verification of work experience and proof of his or her legal right to work in the U.S. Applicants who cannot provide such information tend to eliminate themselves at the outset.
This approach is easier for the applicants, and it prevents their having to bother previous employers unnecessarily. Previous employers should be asked for such references only as one of the final points in the screening process.
This is a tight labor market, and most employers today are trying to treat applicants professionally and comfortably. When employers do unusual things during the hiring process, it is unusual for the better applicants to stick around.
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