I think I get it. Employers are required to accept any VALID set of documents that are specified on the I-9 instructions. To demand a green card (as opposed to some other set of documents that might validate a legal immigrant's work status), may be a violation. It actually says this right on the I-9.
For example, as a citizen, I can show my driver's license AND social security card, OR just a passport.
I believe you can have a social security card and driver’s license while you have a green card, without being a citizen. I believe I had both when I had a green card, before I became a citizen.