More succinctly, just refusing entry to those from Syria and Iraq wouldn’t have prevented 9/11 or the SB massacre.
And if we include nations with an ISIS presence we inadvertently include our own allies.
Of course not. But it would prevent the entry of any infiltrators known to be present in the group of refugees that are an immediate concern.
And, secondly, limiting the banned countries to the two o immediate concern increases the chance of passage. A comprehensive list of all risky countries wouldn't have any chance of surviving a Democrat filibuster.
And if we include nations with an ISIS presence we inadvertently include our own allies.
As I noted earlier in a post concerning French Muslims, we have to trust our allies to vet their own citizens. If we didn't honor a French passport -- Muslim or not -- we'd run the risk of retaliation against American passport holders. Or trade.
In other words, we'd be playing with diplomatic fire. Imagine how we'd respond if one of our allies refused entry to an American citizen because he was black.