Can you provide a link?
De-facto it is being used as a national ID card, and not just by government - by various private enterprises. The ultimate evil is EXPERIAN that collects all personal data about every person. Guess what they use for their database indexing? SSN.
Call it whatever you want. It is a federally-issued card that proves one's citizenship and identity. It seems that you assume all national ID cards can only be mandatory, but it's not the case. They don't even have to be called National ID cards - if you remove from the passport card annotation "valid only for land and sea travel ..." and include a holder's signature, there will be no difference between, say, German personalausweis and the US passport card (except for being optional). If some people think that national ID card can be more convenient than state-issued ID/DL card, why not let them have it? It's not like you are forcing it on the rest of population.
Please see INA 274A [this is the statutory source of E-Verify and the form I-9. you can find it on www.uscis.gov on the laws tab....I'm not going to dig for the SSN and passport statutes but you can have fun looking if it really matters to you.]
(c) No Authorization of National Identification Cards.-Nothing in this section shall be construed to authorize, directly or indirectly, the issuance or use of national identification cards or the establishment of a national identification card.