Because membership in a foreign organization is not a crime ... certainly not a crime over which the U.S. has jurisdiction. Thus, with respect to your run-of-the-mill al Qaeda terrorist for whom we have no evidence of the commission of a crime over which the U.S. has jurisdiction (such as an attack within the domestic territory of the United States or, possibly, on a U.S. citizen abroad), release would be mandated under a try or release policy. Obviously, you cannot win a war by letting your enemy walk. The whole idea is to render him hors de combat, i.e. out of the fight. Further, such a policy would encourage our soldiers to kill enemy they encounter rather than doing the more humane thing -- capturing them -- so as not to encounter them again in the field of battle. It's a fool's errand all around.