I am just curious. How could a "national standard for the issuance of state ID's" possibly be "voluntary"? I cannot envision what you are describing.
If you pass or enact a "national standard" for state IDs, that part is not so voluntary (for the states). Or do you mean "voluntary" in the same sense that the state ID's - driver's license, usually - are "voluntary" for people? (I.e. "voluntary" unless you actually want to drive a car anywhere, or live any sort of normal life....)
But even that doesn't sound to "voluntary" to me in the first place, except in a highly technical sense.
No offense, but it just seems like supporters of this or that ID card plan like to throw in the word "voluntary" just to make it sound more palatable, when in reality it has no basis in fact.