There have been, what, seven confirmed Ebola cases in the U.S.? Of those five have been people who traveled to the U.S. with Ebola, and of those four have not had passports from Liberia, Sierra Leone, or Guinea. You can ban people with passports from those countries but they aren't the only carriers.
Source?
Where were their passports from, then?
Any country that allows flights out of those countries is also run be crazy people, by the way.
A legal traveler visiting Sierra Leone would have a visa & entry/exit stamps on their passport no matter where they came from.
passport is simply shorthand for a review of that document for if, where, and when it had been stamped by an infected country.
In view of the potential pandemic and incubation period it is only reasonable to take any and all steps to limit the risk to our own citizens.
Which brings me to the current reports of a medical worker whining about being isolated by NJ upon return from W. Africa: seems she feels all righteous about tending to ignorant tribes elsewhere but has no concern for her own countrymen. (No, "tribe" isn't racist and "countrymen" isn't sexist. "Savages" would apply but invite misrepresentation and "countrypersons" sounds dumb.)
Try taking a vacation to Israel with a US passport, then flying from there on to Saudi Arabia.
Visas and Entry/Exit stamps matter, too.