A passport is not required to go
to the USVI from 48US/AK/HI/NM/PR/AS/VI.
USA (US)
Passport Exemptions:
- When arriving directly from the US mainland, Alaska, Hawaii,
Northern Mariana (Isl.), Puerto Rico, Samoa (American),
Swains Isl. or the Virgin Isl. (US) to Puerto Rico/US Virgin
Isl., a passport is not required.
(emphasis added)
So the carrier can't be fined for transporting anyone to USVI from 48US/AK/HI/NM/PR/AS/VI.
But coming
from VI to 48US/AK/HI/NM/PR/AS does require a passport except for US Nationals and LPRs (a point on which IATA and CBP diverge somewhat). IATA says:
National USA (US)
Embarkation Virgin Isl. (USA) (VI)
Destination USA (US)
Passport required.
- Nationals of USA must hold passports and/or passport
replacing documents valid on arrival.
CBP says:
Will travelers from U.S. territories need to present a passport to enter the United States?
U.S. Citizens and Lawful Permanent Residents (LPR's) who travel directly between parts of the United States, which includes Guam, Puerto Rico, U.S. Virgin Islands, American Samoa, Swains Island and the Commonwealth of the Northern Mariana Islands (CNMI), without touching at a foreign port or place, are not required to present a valid U.S. Passport or U.S. Green Card. However, it is recommended that travelers bring a government issued photo ID and copy of birth certificate.
(emphasis in original)
(More amusing is the language "required to present"; I believe the USC provides than an LPR must always have his green card on him and present it upon demand . . . except not to CBP in USVI . . . according to CBP. Very strange; an agency website doesn't supercede the USC, of course, but whatever).
https://help.cbp.gov/app/answers/det.../980/kw/virgin
Presumably, there is a CFR that deals with this in more detail.
In CBP lingo, a sentence starting with "it is recommended that . . ." might just as well start with "the law does not require but we will make your life a living hell to the best of our ability unless . . ."
For example, CBP "recommends" that Citizens answer their questions on arrival.