That makes sense in a vacuum, but if some of, or all of the principal actors of a given institution run a tight ship it's entirely probable that there are various unknown secrets to which only they are aprised. There's nothing that forbids some key officer from literally burying information in a completely undocumented spot. In such a case, if the buried treasure is of any value whatever, it becomes a point of leverage. Some artifact that could be lost to the world forevermore or discovered, hinging on some nefarious negotiation.
Not to mention the reality that aside from some sense of justice, allowing any of these people their freedom is probably irrelevant in the larger scheme of things. Which is to say that it's a gambit for valuable, empirical knowledge which for all intents and purposes, I would surmise, couldn't be elsewise obtained due to the moral standards of the West, or some theatrics which will have little consequence.
Documents have a habit of burning to ashes if they have a chance of becoming evidence.
What this is referring to is the relative immunity Unit 731 got post-war in exchange for research results being handed over, which is a well-known historical fact.
It does make a lot of sense. In the most basic terms, almost no American spoke Japanese, let alone at a scientific level. Collaboration would be at the essence.
But I think the greater point that you are missing is that you can't walk over a country of 150 million people and achieve total power. Without large doses of good will, collaboration, and soft power, resistance gets in the way of every single goal an occupation has.
Not to mention the reality that aside from some sense of justice, allowing any of these people their freedom is probably irrelevant in the larger scheme of things. Which is to say that it's a gambit for valuable, empirical knowledge which for all intents and purposes, I would surmise, couldn't be elsewise obtained due to the moral standards of the West, or some theatrics which will have little consequence.