| Is it actually illegal to reenter the US as
  | a citizen without a US passport?

I've talked to the border patrol over the phone about this before (with relation to re-entering with an expired passport). As an American citizen, you can't be denied re-entry into the country. Without proper documentation, you might get tied up at the border while proving you are an American citizen (though I'm not sure what lengths they would go to to figure out if you really are a US citizen without even an expired passport though).