Interesting, what exactly are they going to do if someone doesn't have one? IANAL, but as far as I recall from "what if they want you to unlock your phone/mount your encrypted drive/... on the border and threaten to not let you in" articles that purported to be written by lawyers, they cannot not let a citizen in.
Answering my own question... CDC page has better information - https://www.cdc.gov/coronavirus/2019-ncov/travelers/testing-... this is basically at the point of departure, so the airline won't let you board in the first place, and the awkward issue of not letting citizens into their own country won't arise.
Most places in the world don't have reliable means of getting a test done within the 3-day return time. I guess when I go visit my family abroad, I am going to fly back to somewhere like Tijuana, cross the border, then fly domestic back home. I'd encourage as many people as possible to do the same.
