As has been explained many times but some people aren't listening, the ban would be temporary in order to discover and implement improved vetting.  Once again, I will explain.  Not all countries have a reliable vetting process for the people they allow to leave their countries; they may also not have the technology/databases to keep track of criminals, institute watch lists, and other modern surveillance processes.  Countries with Muslim theocratic governments are  suspect since Islamic terrorists tend to be born, raised, trained, or traveling through Muslim theocratic countries.  If you want evidence, you may need to wait until your shopping mall blows up with you and your family in it.  Would that be enough evidence?  I know that sounds harsh, but the point is to do the very best we can to determine what kind of people we let into our "home" and how they are going to behave when they get here.  We are under no legal or moral obligation to accommodate every person in the world.