When you don't know for certain you err on the side of caution.
Proof may get you killed.
I don't understand why they brought any ebola victims to the US. Was that supposed to be macho?
They should be sending volunteers to Africa to try and stop it. The more people who get infected the higher the probability that random mutation could cause it to evolve into an even deadlier form. It may be a low probability but what is the point of taking the risk? It had to evolve into existence after all.