CBP and CDC could delay admitting the person and wait to cherry pick on something as a potential symptom of some public health threat and then latch on to that to mandate testing to see if quarantine/isolation is necessary or not:

https://www.dhs.gov/news/2014/10/08/...elers-airports

Some would perhaps call the result a quarantine/isolation anyway.

Check this out too about “Federal and State Quarantine and Isolation Authority”:

https://fas.org/sgp/crs/homesec/RL33201.pdf

It’s about more than just quarantine and isolation authority, but it covers that too.