O Canadá decide o que é necessário para entrar no Canadá; os EUA decidem o que é necessário para entrar nos EUA. Seus governos tomam essas decisões de forma independente e não necessariamente precisam concordar.
A exigência de passaporte dos EUA para os cidadãos foi criada como parte da Reforma da Inteligência e Lei de Prevenção ao Terrorismo de 2004 (IRTPA), Lei Pública 108-458 . Veja Seção 7209 (b) (1):
Development of plan.--The Secretary of Homeland Security, in consultation with the Secretary of State, shall develop and implement a plan as expeditiously as possible to require a passport or other document, or combination of documents, deemed by the Secretary of Homeland Security to be sufficient to denote identity and citizenship, for all travel into the United States by United States citizens and by categories of individuals for whom documentation requirements have previously been waived [...]
Logo acima disso, o Congresso explicou por que eles estavam adicionando esse requisito:
Findings.--Consistent with the report of the National Commission on Terrorist Attacks Upon the United States, Congress makes the following findings:
(1) Existing procedures allow many individuals to enter the United States by showing minimal identification or without showing any identification.
(2) The planning for the terrorist attacks of September 11, 2001, demonstrates that terrorists study and exploit United States vulnerabilities.
(3) Additional safeguards are needed to ensure that terrorists cannot enter the United States.