I heard the following on talk radio today and confirmed it with a NYT article:
Duncan rented a room either from the Williams family or the neighbor of the Williams family. The 19yo daughter was stricken with ebola; seven months pregnant she was convulsing. Duncan helped carry the daughter to a taxi after they couldn’t get an ambulance. Sitting in the front seat he accompanied the girl and her mother to the ebola center, and then carried her into the facility, but were turned away because the center was full. He carried her back to the taxi, and back to her home, and then left town a few days later (one could argue, to seek American medical care). He did tell the intake nurse at the hospital that he had traveled to Liberia/West Africa, but that information was not passed up the line. (whether he then told the doctor or not is an unanswered question)
He should have told them way more than just that he traveled to West Africa - like “A FEW DAYS AGO I CARRIED A VERY SICK EBOLA VICTIM TO THE CLINIC.”
He simply could not have made the decision to come to the US after being probably exposed.
The ticket was purchased two weeks before he traveled and therefore long before exposure. The process for getting a visa probably started at least two or three months before that.
So when he got on the plane he was simply implementing longstanding plans.
Though arguably he should have made the decision not to travel for fear of contaminating unaffected people. Although it's unclear why we should expect individual foreigners to impose travel restrictions on themselves the US government isn't bothering to impose.