b might not be so insane if you consider that tumblr is used by a lot of people who posts selfies. That would include a lot of teens/tweens. The problem with selfies is that they wouldn't be in anything like photoDNA and thus impossible to catch by any of the modern day algorithms except for the ones designed to catch porn in general. They might not have any better solution (of course they would ban stuff as it was reported, but that might not be good enough) and apple's move forced their hand. Guess the question is what caused apple to do what it did now.