[jsmith45]

Right, but the only punishment for failing to comply is reprimand and delay at the entrance while trying to verify citizenship. The latest publicly available CBP Inspector's Field manual is unequivocal that if the Inspector is convinced the entrant is an American citizen they cannot stop them from entering.

The law really exists as a way to avoid delays at entry as a US citizen having a valid US passport can be processed in much faster than one who does not, and it minimizes the risk of being stuck with an immigration agent that does not belive your claims of being an American citizen due to lacking convincing records and documents.

American Citizens are not the only ones with an unequivocal right to enter. American Indians born in Canada who have are at least 50% American Indian by blood "cannot be denied admission to the United States". They are required to prove to the inspector that they meet this criteria, but if they do, they don't need a passport or anything. (But they probably needed some documents to convince the inspector of their status as a 50%+ native born in Canada.)