More like mis-informed assertion on your part. The filing employer is REQUIRED BY LAW to prove that they couldn't find an equally qualified person in USA.. They are required to post a job listing and vet applicants. The H1B abusers like Microsoft, Infosys skirt around this by putting an ad in an obsure local newspaper that no-one is going to read. And even if someone applies the Hiring Manager finds some grounds to reject the applicant without legally getting into a pickle ("ex: candidate failed verbal tech interview etc).
> They usually leave the H1B Visa workers intact
For filing an H-1B an employer does not need to "file justifications for not being able to find an equally qualified person in USA".