Doesn’t matter. Even if a passport isn’t from the countries in question, they are stamped if they’ve been there.
There’s no way that custom officials did not know that all of those folks had been in the hot zone.
Therefore there is no excuse.
I haven't been to West Africa recently, but it used to be that stamping passports was a hit or miss thing. Many stamps are illegible, skipped, smeared, or otherwise not obvious evidence of travel to that country. Still, stopping the issuance of new visas, cancelling existing visas, and checking for passport origin and stamps (do you know how much time that would take, since pages are not stamped neatly and in order?) is a good start and should be policy. BTW, it won't happen. To this White House, it's more important to pretend that open borders are a good thing, so Obama can let in more undocumented democrats, than it is to protect the lives of Americans.