an icepick through the keyhole

Further, the danger is that such prosecutions will be politically motivated, i.e. raised because of anti-American feeling, rather than objective indictments based on evidence. [...] Surely the scrutiny of world opinion would ensure that such actions are above board?

I think you just answered your own question. Yes, the scrutiny of world opinion would help ensure that the rules of war are followed -- but Belgian courts (using Belgian law) are not a manifestation of world opinion. They are, rightly, not going to accept meddling in their courts any more than we would in ours, so they (and we) are not the appropriate venues for resolving such matters.