It's got nothing to do with stupidity. If someone's brother gets murdered, and you suggest that his actions were somehow responsible - i.e. walking around in questionable neighborhoods in the wee hours of the morning - you may get dirty looks or even a knuckle sandwich. 9/11 is something like that multiplied by 3000 times. People are enraged. My advice? Don't go there.
If my Brother was killed, I would take out retribution on their brother and leave it at that; that is retribution. I DO NOT have the right to kill their brother in addition; that would be an act of aggression. Bringing the analogy back to 9/11, we slapped around the man who killed our brother, left him bleeding, and started beating the crap out of his second cousin.
Very true, 9/11 is way too charged and candidates (ie Giuliani) will jump on that in a heartbeat... But if you listen to what Paul said and the way he said it, he cares about this country and firmly believes our policies are weakening us instead of strengthening us.