Why would you think that they were more humane?

The Japanese were only at war with the USSR for the last couple months of the war. Most of the Japanese and Russian interests in the north Pacific were settled by the Treaty of Portsmouth in 1905 so they had no real reason to declare war on each other. The cost of a Japanese invasion into Siberia would be much worse for the Japanese than the Russians. Freezing winters, very sparse population, lots of rivers, etc.