Wasn't there always the exception that American citizens could still return form wherever? And it's been said often that there is a huge amount of travel back to China for the Chinese New Year (January 25th this year). There was probably a large number of Chinese-American citizens going back and forth, plus US citizens who were in China for other reasons.
this is true - dual citizens and green cards were allowed back in. After DPrincess event, all cruisers should have been mandatory quarantined. Email followup may have been inadequate to keep returning travelers home. And, because we were unaware of person-to-person, there seemed less urgency in the public. Lesson learned in hindsight - contrary to WHO and CDC advice, travel restrictions do work and are effective but quarantine of incoming is an essential element to control.
I have a hunch that if we cut out Chinese traffic during Jan/Feb, or put all returning Chinese New Year travelers and all cruisers on mandatory quarantine, we’d have a lot less ‘normal flu’ cases each year - in and out of nursing homes.