“In my opinion, yes, it’s controversial, a little unethical,” said German Silva, a two-time Olympian and now a top Mexican distance-running coach.But it is legal. In fact, top athletes crossing borders to enhance their Olympic prospects are part of a well-established pattern that involves many nations and events. Four years ago at Athens, Americans of Greek descent took the field for the host nation’s baseball team, a Chinese-born table tennis player competed for the United States and a Jamaican woman sprinted for Slovenia.