Why Americans don't need a visa to go to Canada?

Canada's visa policy mainly depends on Canada's diplomatic relations with other countries. People from many Commonwealth countries can get visa-free treatment when they need to enter Canada, and the United States is one of them.

Citizens of any country are allowed to stay in Canada visa-free for six months as long as they have legally obtained permanent resident status in the United States, or have a green card in the United States, or can provide permanent resident identification. Under normal circumstances, tourists can stay in Canada for up to 6 months. However, if tourists have special needs, they can apply to Citizenship and Immigration Canada for extension of stay.