My Hist. Teacher says that it was because surrendering was considered "dishonourable" by the Japanese.
There is, as with most interesting questions, no one or simple answer. The surrender element did play a part, and indeed the IJA indoctrination – steeped as it was in the hypernationalist quasi-neo-bushido-ism of the time, regarded surrender as highly dishonorable and indeed, as is well known, mandated that their own soldiers fight to the death rather than surrender, as per the 1941 “Code of Battlefield Conduct”.
But it’s also something that was more fundamental to Japanese social order of that era, and even forward into modernity. Though there were indeed reported incidents of Japanese soldiers and guard treating prisoners with respect, or of “sharing a burden” with their charges, on the who the Japanese mindset was heavily indoctrinated to the idea of a strict, severe hierarchy at every level of society, and pertaining to every relationship – personal, professional, or otherwise. Commands given by superiors were to be followed, period, and there were extremely harsh punishments for subordinates who failed to adequately carry them out, including physical beatings and public humiliation. Tom Uren writes, “Japanese military discipline was sadistic, because they administered instant or Japanese punishment. This was carried out on their own troops, but when it was administered to prisoners it was particularly vicious and brutal.” There was very little in the way of recourse or complaint towards superiors who gave unreasonable orders, or abused their positions of power, which naturally gave rise to a particularly high level of stress on each subordinate level of the command structure as the orders worked their way down the chain. It’s not difficult to see where that frustration would have often been vented – upon your own subordinates, whomever them may be, if for no other reason than to release one’s own pent up frustrations. This is not, of course, a uniquely Japanese phenomenon – indeed the German Wermacht was likewise held in ignominy by the Allies for their similar draconian treatment of surrendered populations.
POWs and captured populations, then, fit into this social equation as the very bottom, powerless rung of the ladder. After all, they were obviously honorless captives, and completely at their guards’ mercy. This provided and exceptionally easy target for any and everyone who might wish to vent their personal frustrations onto a person or people who could not fight back. Again, this is not uniquely Japanese – we see it time and again in every military conflict: abuse of prisoners. Of course, it was not helped by the fact that Japan, though a signatory of the 1929 Geneva Conventions on Human Rights, had failed to thereafter ratify it. However the branch of the Army tasked with POW management was about the most dead-end career track imaginable. It was for the people who couldn’t hack it in better regiments, or sometimes were simply uniformed criminals, and they were constantly disrespected by their superiors as a result – as you can imagine, the “Crabbes and Goyles” of the IJA turned the humiliation and frustration on the most convenient punching bags there were: their own helpless charges.
There’s a psychological aspect to all of this – if your captors are capricious and unpredictably violent, that tends to heavily discourage resistance. Whether or not that was understood at an institutional level is an interesting question that I haven’t the answer to, but undoubtedly the not only brutality but unpredictability of that brutality made large swaths of the conquered populations and far more cowed than they might have been under a “gentler” hand. Ray Parkin, who was himself a British POW to the IJA wrote of his experiences and noted the “fatal impulsiveness of the Japs [that] can so readily show.” Any perceived infraction might be responded to “with a fist, hammering over the face and head with wooden clogs, repeatedly thrown to the ground…kicking in the stomach and scrotum and ribs etc ... When the men fell to the ground, they were somehow got to their feet by such painful stimuli as the above and the dose was repeated”… whereas the following day that same guard might simply laugh the same infraction off and offer the prisoner a cigarette. It was a condition of perpetual terror, which paradoxically enough tends to breed servility rather than defiance. That is no ringing endorsement of the practice, of course, and even from a solely military sense, it would prove to be far more damaging for the IJA in terms of collaboration and information-gather from prisoners, than it was helpful in terms of potentially staving off rebellions.
This understandable (if horrific) practice was furthered by the ongoing and highly effective indoctrination of the IJA. From Max Hastings, “The Japanese treatment, not only of their military prisoners but also civilians, represented this very fundamental aspect of Japanese military culture that far from displaying respect or mercy for the weak, the weak deserved to be treated with contempt. Only strength was valued, only strength was admired. In some ways this was a cult not very dissimilar from that of the SS, the belief that people who were weak enough to surrender and especially who had been weak enough to surrender without even putting up much of a fight deserved to die and deserved to be treated as animals and as slaves.”
Another aspect of this, though, was almost completely accidental. The IJA was, to put it briefly, surprised by its own level and scale of success, and the rapidity with which that happened in the first several years of the war. Many colonial posts and bases were taken by surprise, and as a result a large number of them simply surrendered with more or less their entire contingent of troops intact. The IJA planners had not anticipated this – nor the large number of surrendering soldiers they suddenly found themselves in charge of. Moreover, the IJA command was overly focused on the goings on of the front line, and throughout the entire war on all fronts paid far too little attention to their own internal logistics, supply lines, or resource management for its own occupied territories. Given the relative lack of regard for soldiers surrendering without a fight, and their own dearth of supplies with which to adequately take care of them, it was deemed expedient to simply march them around until fatigue and malnourishment whittled their numbers down: hence the death marches.
Dower, John W. (1986). War Without Mercy: Race and Power in the Pacific War
Parkin, Ray (1963). Into the Smother