Uh, well I would agree with you that he is an "odd little dude," but not because he runs for America, but because he has weird pre-race rituals.
I guess to answer your question and assume you aren't trolling: Leo runs for America because this is the country he wants to run for and he is an American citizen? I'm not sure why this is even a question, really.
A cynic might say that he runs for America because he gets more sponsorship money that way than if he ran for Mexico.
At any rate, the answer is certainly one of those two, I suspect the first.