It's a case of wanting to play soldier by outdated rules and concepts. The rules by which the Brits tried to punish Doenitz were for use during WW1, and even then were vastly outdated. The Prize Rules covered such things as warning the target vessel of the impending attack (a foolish concept if ever I heard one), aiding and assisting any survivors of such an attack. It even states that a submarine must stop the the target vessel, board and search it to determine it's status. Further details can be found here. Such rules were completely impractical in an era where the successful use of submarines depended on secrecy, and the precedent established by Doenitz and Nimitz ensured that those rules were thrown out after WW2.