What this neglects is that Saddam Hussein, in contrast to al-Qaeda, is completely exposed to US retaliation. Unless one believes that Saddam is suicidal, there really isn't much of a spectrum of possibilities here: either Saddam had only the most tangential, almost accidental involvement in 9/11, insufficient to provoke a US response, or he is fully calling the shots and has his own controls in place to deter our retaliation. Ask yourself which possibility better fits the observed facts to date.