Final answer:
Joey could use a U.S. passport to travel to Japan because as a U.S. citizen, either by birth or naturalization, he is legally required to use a U.S. government-issued passport for international travel.
Step-by-step explanation:
The legal reason Joey could use a U.S. passport to travel to Japan is likely because he has obtained U.S. citizenship, possibly by birth or through a naturalization process. By law, all U.S. citizens must hold a government-issued passport to enter or leave the United States, as mandated by the Immigration and Nationality Act of 1952. If Joey was born in the U.S. or has been naturalized as a U.S. citizen, he would be entitled to use a U.S. passport to facilitate his international travels.