According to the Department of Homeland Security the following airports are considered under US regulations:
BAHAMAS
Nassau, Bahamas, Nassau International Airport
Freeport, Bahamas, Freeport International Airport
Paradise Island, Bahamas, Paradise Island Airport
GUAM
Tumon, Guam, Guam International Airport Terminal
PUERTO RICO
San Juan, PR, San Juan International Airport
Aquadilla, PR, Rafael Hernandez Airport
Culebra, PR, Benjamin Rivera Noriega Airport
Fajardo, PR, Diego Jimenez Torres Airport
Mayaguez, PR, Eugenio Maria de Hostos Airport
Ponce, PR, Mercedita Airport
San Juan, PR, Isla Grande Airport
San Juan, PR, Luis Munoz Marin International Airport
Vieques, PR, Antonio Rivera Rodriquez Airport
VIRGIN ISLANDS
St. Thomas, VI, Cyril E. King Airport
St. Croix, VI, Henry E. Rohlsen Airport
Bearing in mind this fact game restrictions for US based airlines should be checked in order to let any US based airline expand operations as hubs or terminals in the above mentioned airports, like in real life airlines such as AAL and COA do.