Neither is Elon Musk. And the SpaceX team is multinational as well, talent knows no borders.
[1] http://www.esquire.com/news-politics/a16681/elon-musk-interv...
Source? He is a US citizen so I'm not sure what 'American' would mean otherwise.
I imagine its also because "SpaceX" is awkward to chant. Chanting too fast or with too many people and it will start sounding like "Space sex". Don't believe me? Say "Space X" ten times fast...