The situation you describe is all well and good in a world where people aren't shooting at each other, and I would, in fact, agree with you if that were the case. Once hostilities commence, things change. There are, as I see it, a few different ways of looking at who has legal authority at the time of Mr. Hicks' capture.

1) You could say, that from some fixed point in time after hostilities commenced until some point in time when hostilities cease (which they still haven't) there is no effective governing body and therefore no rule of law, period. Obviously, no crime could have been committed since the required legal establishment to enforce laws ceased to exist at pick some point in time and wasn't able to reassert itself until pick some later point in time.

This would be a very dangerous precedent to set though. What if tomorrow it turned out that a squad of coalition soldiers gang-raped a group of school girls the same day as Mr. Hicks was captured? They'd be able to use the "no rule of law" defense just as easily as he could. So how would we prosecute them?

2) You could say that the Taliban was still in control at the time and as such only the Taliban has a right to prosecute. That would be akin to saying that only Nazis could have held the Nürnberg trials. Mr. Hicks was captured allegedly acting on behalf of the Taliban. A Government which no longer exists as a legislative body.

3)You could accept that traditionally held view during warfare that "He whom holds the ground is in control and therefore makes the rules for as long as he can continue to hold the ground." Not only is this the age old and traditionally accepted way of doing things --and exactly what has happened here-- but there is even a term for it... Martial Law. Precedent is so deep here, I'm not even going to start citing examples so that I can keep this post to a readable length.

4)It could be asserted that only the current Afghani Government has the right to prosecute. If that's the case though, wouldn't they also have the right to turn Mr. Hicks over to the U.S.?

5) As an Australian citizen, Mr. Hicks is only subject is to Australian law in Australia.

This would effectively give diplomatic immunity (hmmm...I wonder if there is a reason this only applies to diplomats? Maybe because it doesn't apply to other foreign nationals who commit crimes on host nation soil) to every citizen of any nation who committed a crime in a country other than his own.

Those are five ways I could see it argued. If you have a point of view different from one of the above, I'd love to hear it. Because quite frankly, I think your claim to him is dubious at best, and ridiculous at worst.