[wisty]

There's no single answer.

In Palestine, most of the terrorists (but maybe not the leaders) are alturists. Israel has captured quite a few, and profiled them - http://utexas.academia.edu/AmiPedahzur/Papers/209890/Altruis.... The same may have been true of the IRA, and it may be true of the Tamil Tigers. People only blow themselves up when they believe they are fighting against evil oppressors. In these cases, you are right - you need to address the root cause.

In the case of 911, the terrorists themselves may have been altruistic, but they were given support by Afghanistan (and maybe a few Saudis, but we need their oil). Knocking out states which support terrorism (which the US can do quite easily) may be an effective way to stop state-sponsored terrorism.

In any case, it's partly the result of peer pressure. If everyone in a community sees terrorism as a good thing, a few people in that community will become terrorists. Threatening states forces them to fight this attitude. In other cases, there is no state to threaten.