A good way to treat CSAM is to see it as radioactive material. You always want it to be accounted for. Some people have legitimate use for it (x-ray machines, nuclear power plants, etc...) these would be the doctor example in this article. and then you have the actual illegal uses of it, like someone trying to build a bomb in their backyard. In that sense, a CSAM filter shouldn't assume anything about the content, just be able to identify that its content that needs to be accounted for.