I suspect it has to do more with human moderation. As much as Google hates using humans for anything, moderation still has a human element. As low as the minimum wage is in countries with a workforce educated enough to do this unpleasant work, it still costs something. Drawing a line in the sand and saying "your videos are so unlikely to generate views that we won't even spend a couple of cents making sure it doesn't violate our policies" probably saves them several millions per year.