Green card job postings do this because it is a requirement to advertise the opening. So you tailor the job description specifically for the person you already employ on a immigrant visa.
There's not really any green card job postings. But getting an H-1B can be the first step toward obtaining a green card (permanent residency) for some immigrants.
And still don't hire anybody who shows up who actually has those qualifications.
And isn't that how it's supposed to work: hire local in preference to bringing in someone from outside? I'm an immigrant myself and I still think that's a good idea.