All? No. The NBA consists of multiple nationalities. However, judging this by the nationality of the MVP candidates is silly in my opinion. Canada doesn't even have a stranglehold on it in the NHL. The USA is to basketball as Canada is to hockey. We produce the most NBA talent, and looking at an article about non-US players on opening night rosters this season, there were 125. The NBA has a roster limit of 15 per team, so multiplied by 30 teams in the league, that leaves 325 US players. We're also the dominant force in international basketball, similar to how Canada is the dominant force in international hockey. Slightly more so even, with 17 out of 20 Olympic men's gold medals going to the United States, including the last four in a row, as well as 8 out of 9 where NBA players were allowed to participate. The women's side is similar, with 10 out of 13 gold medals going to the US women.
The gap is, however, closing. The 1992 men blew out everyone en route to gold, but the games are now much closer, and the US women beat France in the gold medal game by a single point in the most recent Olympics.