Bush is simply waging this war on his own terms, and in this case it's based on what I call playground justice. You remember the bully in school who always did stuff behind the backs of authorities, trying to get you in trouble. Well, where I went to school, the bully usually got his comeuppance on the playground - a place where, once again, you could get him out of view of the authorities. The bully had already set the rules by going behind the back of the teacher, so he got his later.
The Taliban is going to get its @$$ kicked, but an official declaration of war is like seeking justice in front of the teacher (or, in this case, the Geneva convention).