Oh, hadn't you heard?? Duncan got his visa in August, thanks to the administration's change in policy to streamline visa requests from west Africa (implemented after ebola hit the urban centers):
http://townhall.com/tipsheet/katiepavlich/2014/10/17/wow-obama-adminstration-started-streamlining-visas-from-west-africa-in-august-at-height-of-ebola-crisis-n1906576
I still haven't seen anything that would indicate Duncan's visa was affected by the policy. In fact, the wording kind of implies that his visa was approved before it went into effect.
Here's an approximate timeline.
June, he starts applying for a visa.
Late August, visa approved.
September 2, ticket purchased.
September 4, quits job.
September 15, exposed to Ebola. The people dealing with the girl reportedly thought she was having complications of pregnancy.
September 18, Duncan leaves Liberia.
The timeline simply doesn't allow for him deciding to come to America after knowing he was sick.
http://www.thedailysheeple.com/the-tangled-timeline-of-thomas-duncans-last-days_102014