easy fix for the prosecution: Use parallel reconstruction. I'm sure they're on this as we're commenting here.

Once the information has been gathered, it's out there. You can't unsee it. It will always be there. Now, I don't think we should protect child porn users, but this is a general issue with mass surveillance. Laws change and suddenly somebodies actions that were once legal and are now illegal will be useful for the investigators as a basis for parallel reconstruction or just plain extortion or blackmail.