Here is my theory on the Dallas guy.
He had applied for and received a visa to visit family in the U.S.
He had helped a pregnant neighbor who became ill. When he found out she died of ebola, he got scared, knowing that the only place he might get cured if he came down with it, is in the U.S. He lied to the people in Liberia at the airport about being exposed to ebola. I wonder how honest he even was with his family about being exposed to the ebola.