NBA (National Basketball Association) was founed in United States, in 1949, after the merge of Basketball Association of America and National Basketball League. Before NBA expanded to Canada in 1995, it consisted of all American teams. In 1995, 2 Canadian NBA teams, Grizzlies and Raptors, were formed in Vancouver and Toronto, respectively. Toronto Raptors is the only non-American NBA team now.
Note: National doesn't imply it's restricted to it's own country. NBA is a business anyway.