I don't believe it's stupidity.
It's weakness. He does not want to suffer the incoming fire FROM EVERY DIRECTION.
Or, it could be that he just plain believes it's wrong.
But I'm going with weakness.
Weak is fine.
But it makes no sense given the other things he’s said that this isn’t entirely logical.
Trump has followed the logic to its natural conclusion.
If you don’t want Syrian terrorists granted entry, then you don’t want any terrorist granted entry.