But, in some cases,the “reports” harmed innocent people, ironically including the child who was nominally to be protected.  British News anchor Julia Somerville’s case may be the most prominent (http://www.independent.co.uk/news/julia-somerville-defends-innocent-family-photos-1538516.html) but there were others.  There was even a made-for-TV movie about the problem, Snap Decision (http://www.imdb.com/title/tt0286132/)

Furthermore, there is a lot of stuff on a modern computer that owners don’t choose to put there.  We all get spam, and some of that spam has attachments.
We install software Some of us encounter malware that uses our computer in ways we don’t intend.

Finally, what vetting is there of the hash list used by the Cloud provider?  What assurance does anybody have that the hashes on it each indicate the presence of child porn?  If the list isn’t accurate, then a hit would not provide probable cause for a search.