I don't disagree and more than likely most immigration could be stopped from the hot zones, well unless they went through Mexico first.
The question is who is going to got to the hot zones and keep the people from immigrating elsewhere and spreading the disease to others? Damn near be impossible to ban the entire world from entering the US.
Yet another "If it isn't 100% effective, we should not do it because badfeelz."
mak mentioned the new-fangled flying machines and automated counting devices.
Also, bordering countries are putting up travel bans. So, travel from Liberia in the face of
airborne travel bans will include overland travel to a neighboring country, evading THEIR travel ban, and getting false docs/bribing their way on to a plane. All of which takes time, money, access to other resources. Time during which the traveler can come down with obvious symptoms or just plain die on the way. Either way, that means they don't get on the flight from Lagos or Dakar to London and then to DFW.
This is rapidly moving to the right on the Pareto Chart, addressing less and less frequent/likely events.
http://en.wikipedia.org/wiki/Pareto_chartAnd thus far, we can estimate that for every DEG stopped by the various bans that there are two nurses in America who won't contract ebola with one of them likely dying.
This is not rocket science, folks.