I think that sociology can offer some valuable insights into the existence of such networks, albeit that explanation may not be what moralizers of variaous stripes want to hear. Man (pardon non-inclusive language) is above all a social animal before being an individual. Thus, group norms, expectations, and rules of behavior can explain his (or her) decisions and actions.
Consider, for example, Jack Katz, _Seductions of crime : moral and sensual attractions in doing evil_ which argues that evil acts result from group interaction dynamics. For example, criminal groups value "toughness" - and thus members of those group compete among themsleves for status by proving their toughness by engaging in risky and violent acts, such as killing a person for a few dollars. Those acts cannot be explained from a rational, cost-benefit point of view, since the risk is disproportional to any possible gain. However, they are explained by the argument proposed by Katz - taking the risk is not a means to a material gain (as the rat-choice perspective has it) but a way of proving one's status in a group: the greater the risk, the greater the status gain.
Although Katz's argument is based on empirical observations of criminal behavior in the US, it is easy to to extend it to explain terrorist behavior. From that point of view, terrorist acts are not a means to political or social ends (or even some form of 'social justice' as the loonie left sometimes maintains) - but the end in itself - or rather the way to status and glory in the terrorist world. Any rationally thinking person could easily see that the attack on the WTC would result in a net political loss for the Arab/Islamic world - so no rational calculus can explain that act. However, if the goal is not political gain but gaining status and prestige among like-minded individuals - destruction of the WTC served that goal quite nicely.
What is more, if the above explanation is correct, we can expect even greater calamities to come - as other terrorists will try to outbid the September 11 "achievement."
wojtek