<Perhaps because terrorists are considered criminals in the legal sense, rather than soldiers.

There are governments which support terrorists and terror training facilities. At the moment, the laws of war are the only regulations that govern how another nation may respond to open aggression by foreign-supported terrorists who are not necessarily citizens of the aggressing force, and terrorists are not covered. Terrorists are considered wild-cards to the political arena, and we, as a global community, should require a rehash of the rules of engagement before the game may be properly played. Failing to do this will result in worldwide aggressive action form many nations, with both political and military responses, as would be readily apparent should the U.S. decide that tomorrow was the perfect day to march on Baghdad.

It's time to rewrite a few international laws, and the sooner the better,
Dan