According to the account of the clerks themselves, in spite of seeing threats of murder and carnage, they were reluctant to report what they saw. Their thoughts are best described in this sentence:
"Dude, I just saw some really weird s-," he frantically told his co-worker. "I don't know what to do. Should I call someone or is that being racist?"
Behold the result of brainwashing entire generations with politically correct ideologies! Nobody is to discriminate in the Brave New World, nobody is to be judgemental of others, because, you see, it is racist to consider certain acts wrong especially when another culture sees it differently. The great masses in the West are now borught to the point where their immersion in such an ideology is so great it endangers their very survival. The need to be seen as open-minded and non-racist has all but erased the insinct for self-preservation not only in individuals but in nations as well.
These two clerks, however, managed to rise above indoctrination and prevent the disaster from happening. In another previous occasion there was no such luck. Political correctness saved Mohammed Atta(and doomed others).