I had never heard of this either:
> During the final months of World War II, codenamed Cherry Blossoms at Night, the plan of Unit 731 was to use kamikaze pilots to infest San Diego, California, with the plague. The plan was scheduled to launch on 22 September 1945, but Japan surrendered five weeks earlier.
> Although the Soviet authorities wished the prosecutions to take place, the United States objected after the reports of the investigating US microbiologists. Among these was Edwin Hill, the Chief of Fort Detrick, whose report stated that the information was "absolutely invaluable;" it "could never have been obtained in the United States because of scruples attached to experiments on humans" and "the information was obtained fairly cheaply."[7] On May 6, 1947, Douglas MacArthur wrote to Washington, D.C., that "additional data, possibly some statements from Ishii probably can be obtained by informing Japanese involved that information will be retained in intelligence channels and will not be employed as 'War Crimes' evidence."[10]
https://en.wikipedia.org/wiki/Shirō_Ishii#War_crime_immunity
Basically, everywhere you look you can find these absolute monsters at the helm who drool at “cheaply obtained” and “absolutely invaluable” data from gruesome live human experimentation. Some were just given the opportunity to carry out more of it.