I could be wrong here but, I don't believe Apple has been completely forthright about the purpose of the on device scanning. From what little I know about things thus far, I think Apple is implementing on device scanning with iCloud on, to prevent the sicko from tryin to use the 'I didn't upload that to my iCloud defense.' The scanning on the phone and iCloud probably creates a special account marker that shows the chain of possession and use. In that way, the case against the sick pervert is a little more solid and depending on the action of the pervert, potential distribution charges could be added to the crime(s) involved.