Upvote:3
Traveling to the USVI is considered leaving the mainland and will require you to re-enter the USA when returning from USVI. Essentially, when you leave the mainland, you will need to clear immigration when you return.
You do not need a passport to travel domestically within the United States (mainland or not). Government issued id is acceptable (like a drivers license, state id, etc).
In order to depart on any US domestic flight, you must show a valid government issued id.
To exit USVI you must present either 1.) A valid US passport or 2.) A "raised seal" birth certificate and also a government issued id (like drivers license, state id, etc.)
Nobody is checking what documents you depart the mainland with. It is up to you to have the correct documents to return to the US mainland. They assume you have presented id when you booked your flight to USVI and it is the correct one you'll need to return on.
Nobody is checking airline data for citizenship info. If I held a Russian passport, booked a domestic USVI round trip from mainland and back, I will have to present my Russian passport when I arrive on the mainland.
If I hold a US passport, but travel with my state issued id instead, I will need to have my birth certificate handy upon returning to the mainland, if I do not have my passport on me.
For US citizens, they are not as concerned about entry into USVI as much as the re-entry to the mainland United States.