It comes from the DHS guidelines to States under the Real ID Act. The problem is that it's guidance, it's not actually law. This is what the big fuss in NY was about the DLs and they were going on about issuing them to "illegal aliens", well that was how it was presented in the press but basically what they were on about was issuing DLs to people who had status but the DL would remain valid after the status expired and they wanted to use the "alternative" provisions of the Real ID Act, which in no way means the State is giving proper ID to people who have them.
A lot of States simply amended their laws to basically copy the guidance chapter and verse, only big States like California, Texas and Florida actually bothered to think it through before they did it. New
Mexico is the only State to use the "alternative" provisions of the Real ID Act, i.e. they have a different type of DL that can be issued without proof of legal status, but you can't use it as ID, only as a DL.
This is especially a problem for Canadians because in certain situations you don't get an I-94 like non-immigrant aliens from other countries, so you have nothing to show the DMV. This happened because the guidance simply rambles on vaguely about things States can use to show proof of date of departure.
Florida was sensible about it and created a specific separate provision for Canadian citizens, however States have to be very careful what they do in case they violate the Real ID Act. Until it's tested in court though no-one really knows what is acceptable or not.
My advice is that if you're entering the US for any significant length of time, always ask for an I-94 even if it's not required because sooner or later someone in officialdom will ask to see it.
I bet money that USCIS will amend the I-9 form to specify that only DLs that are issued in compliance with the Real ID Act can be used as proof of identification for employment purposes, they've only just recently amended it to narrow down what types of ID can be used.
Steve.