Why waste their personnel when the markets are likely to fall anyway? In fact, it would probably be counter-productive because then the attack could be used as an excuse for the falling markets. Investors might be more tempted to hold on if they thought it was due to an attack, rather than because the market was falling apart for reasons they don't understand. If you look back at 9/11, the markets were falling apart during the preceding weeks. 9/11 gave everyone cover, and we saw everyone fess up to poor earnings and blame it on 9/11.
A second (third, forth?) attack tomorrow morning would take advantage of the huge negative momentum built up in the past three weeks, demonstrate that the US government has not done anything effective to deter the objectives and potentialities of the terrorists, and bring on political and economic chaos. Note that the slope of the DOW is very similar in September to that of the preceeding few weeks (10320 to 9605, 8/20 to 9/10; 9109 to 8019, 7/1 to 7/19).
If the planners believe that the market will continue to fall, as you think, they may wait for signs of a bottoming (Monday, 9/10 seemed to be a bottoming or reversal day). But weeks like last week, and days like Friday are rare, the odds are that this comming week will not be as bad, and may even be a reversal. We passed a technical trend line that could become a bounce, or a significant signal for a drop to 6500 or 5000 on the DOW.
All pure speculation, of course.