может, у них с собой не было доказательств их гражданства? у нас был бы [certificate of naturalization]…
В субботу идем эпплаиться на мужнин паспорт. 
дополню- не могу правда это утверждать с ссылками - так как это инфа проходила через командование жены - им доводили, что при ВЬЕЗДЕ в США ситизены ДОЛЖНЫ иметь американский паспорт с января 2007-го. Тут парадокс - до 2008 года вроде для ВЫЕЗДА из страны паспорт не обязателен вроде -а для вьезда - обязательно… Там один морячек хотел сьездить в Африку, а у него не было паспорта - вот его командир и предупредил, чтоблы срочно оформил.
http://www.travel.state.gov/travel/cbpmc/cbpmc_2223.html
AIR TRAVEL
Implemented on January 23, 2007, ALL PERSONS traveling by air between the United States and Canada, Mexico, Bermuda, and the Caribbean region are required to present a passport or other valid travel document to enter or re-enter the United States.
LAND AND SEA TRAVEL
The following summarizes information available on the Department of Homeland Security’s website.
JANUARY 31, 2008
U.S. and Canadian citizens will need to present either a WHTI-compliant document, or a government-issued photo ID, such as a driver’s license, plus proof of citizenship, such as a birth certificate. DHS also proposes to begin alternative procedures for U.S. and Canadian children at that time.
SUMMER 2008
At a later date, to be determined, the departments will implement the full requirements of the land and sea phase of WHTI. The proposed rules require most U.S. citizens entering the United States at sea or land ports of entry to have either a U.S. passport; a U.S. passport card; a trusted traveler card such as NEXUS, FAST, or SENTRI; a valid Merchant Mariner Document (MMD) when traveling in conjunction with official maritime business; or a valid U.S. Military identification card when traveling on official orders.
The implementation date will be determined based on a number of factors, including the progress of actions undertaken by the Department of Homeland Security to implement the WHTI requirements and the availability of WHTI compliant documents on both sides of the border. DHS and DOS expect the date of full WHTI implementation to be in the summer of 2008. The precise implementation date will be formally announced with at least 60 days notice.
Note: The passport requirement does NOT apply to U.S. citizens traveling to or returning directly from a U.S. territory.