How do you know? You checked all the US forums?
Pretty well known fact that the top distance runner on the US track and field team is a Sudanese refugee. Actually imported Olympic athletes feature in many many countries teams. The Dutch Tornado sailing team helmsman Mitch Booth is actually Australian by birth. One of the guys on the US basketball team is actually born in UK. It's become quite common for Olympic athletes to adopt a country to play the system and has been going on since the 1980s.