There were rules. Morton claimed that the survivors fired a machine gun at his crew first, so he obliterated them. Since he and Wahoo were lost on 09/11/1943 during her 7th patrol, and given the Wahoo was one of the high scoring US boats (119K+ tons in 6 war patrols), it never came up again until years later. Then, some historians tracked down some surviving crew members (the slaughter was on her 3rd patrol) who gave a very different description of the event then the original.
Basically, Morton hated the japanese and never hid that fact. Since it was a sympathy shared by a lot of command staff too, and since the Wahoo was such a successful boat, nobody was interested in atrocities, alleged or otherwise. Remember, the open, public sentiment at the time was "the only good jap is (was) a dead jap".
|