I hate Kerry as much as the next guy, but this is revisionist history. The "weapons" the attackers got through security were not illegal for air travel. They could have worn them hanging around their necks and not drawn a second glance from security. 9/11 was caused by Clinton's passive response to terrorism on his watch and by our culture of hoplophobia that made the crews and passengers on those planes submit meekly to the hijackers. All the metal detectors and rent-a-cops in the world at Logan couldn't have changed that.
I think the point is, Kerry had very specific evidence of systemic (serious) security flaws and failed to act. Whether or not the weapons were legal isn't really that important. The issue is, if knives can make it through, then why not more potent weapons, and what would a jihadist do armed with ANY weapon? Kerry cannot complain about Bush's treatment of information when he clearly had the same (maybe more specific?) evidence that was tempered further by the urgent determination of security experts.