A quick Google search shows that 28 CFR 29.1 is regulations for the "Motor Vehicle Theft Prevention Act" which allows the attorney general to implement a program where people can put a sticker on their car which means cops can then stop and verify that they're the vehicle owners.
That doesn't mean license plates are optional. Normally I just smirk and move on feeling superior, however they rarely cite something as easy to Google as the CFR.