So I've herd some reasons why the US might wish to target civilians but how 'bout some talk on why AQ and other such groups actively do. I think it has to do with an expanded idea of 'enemy', or at least a broad contempt for the potential casualties, stemming from an elitist world-view; like the believer / infidel distinction in Islam or revolutionary vanguard / everyone else split in Marxism. The identification of them, as less than us, excuses all sorts of violence.

But another reason could be that the group in question stands to benefit from the violence, or rather the hatred, fear and continued violence it provokes. Specifically, in Iraq violence begets more violence but the US needs Iraq pacified. This necessities the continued occupation of the country which directly accomplishes AQ goal of engaging US troops in an extended conflict.

In short, AQ targets civilians because conflict it provokes furthers their goals while the US does not because it needs Iraq pacified to secure its goals.

my .02
Evan