Reef Island is a US Territory which is shown by the magazine on the plane as well as stated by the airplane's captain. They say that US citizens are supposed to go through customs, but this is incorrect. The plane according to Cat went from Los Angeles to Hawaii to Reef Island, thus never leaving US territory. As such US citizens would not need to go thru customs entering Reef Island. They would however, have to clear customs when leaving Reef Island to declare any duty free items.
They show only one US Air Marshal on board. Because of security, air marshals always fly in pairs, and in fact if their partner isn't available, they won't fly. In addition, the plane shown is a propeller which limits its distance and would be a low risk flight not worthy of expending the limited resources of air marshals.