Everyone knows about the PDB, but this is a classic case of revisionism. It's easy to see the warning signs now when you know an attack happened, but you also have to consider the thousands of other estimates, analyses, and predictions that never came to fruition.
Trying to assign blame to 9/11 is the work of a scared fool; it's that same type of fool that uses religion to explain away everything in the world because they are too terrified to acknowledge that there are some things without easily digestible explanations.
A great shot is when you pull it off. A smart shot is when you don't have the guts to try.