Many of the prosecutions are for simple possession of such images. On top of that the "chilling effect" it causes means we can't download large collections of data such as historical web archives (e.g. GeoCities, etc), just in case there are illegal images in there.

The laws regarding child pornography possession are "strict liability" in many jurisdictions. So these laws prevent entirely legitimate activities from being carried out, because of the fear of being persecuted (it has parallels to the Salem Witch Trials).