Frank Shorter (and John McCain and many others) were born to American parents who were on military assignment abroad. One does not have to be born in the US to be born a US citizen, you just have to have an American parent. Which shorter did.
Leo Manzano(and German Fernandez and many others) were not born as Americans, becuase they were not born in the US and neither of the parents were citizens. However, they became citizens at such a young age that it really shouldn't much matter where they were born, they've been American for as long as they can remember.
Lopez Lomong (And Meb K and many others) came here when they were teenagers. In Lomong's case, he did not choose to come here, but both chose to stay. They grew up elsewhere, but didn't do anything with running until they got here. They grew into Americans and have excelled.
Bernard Lagat (and Abdi and K.K. and many others) came here and decided to stay here as adults. They made a conscious choice to come here and another choice to stay here and become citizens. They went through the proper processes, waited their eternity, took their tests and now they're one of us too. Lagat and KK did a lot of their running elsewhere, but does that really matter? They chose to run for us, why wouldn't we welcome them?