The worst argument used by the left against tightening border security

I've heard both Arianna Huffington and Bob Scheer make this argument at Left Right and Center: there's no reason to worry about terrorists slipping over the US border with illegal immigrants some time in the near future, because there's no record that they've ever done it before.

I don't understand why they're so strongly defending the Bush Administration's inaction during the nine months prior to 9-11. Sure, Condi Rice can say that no terrorists had smashed buildings with planes before, but it's not a hard threat to anticipate, people did in fact anticipate it, and the Bushies still did nothing. That's inexcusable, and I don't see why the left is defending his performance.