The problem is that this same technology could be used for different purposes, which are not that honorable as trying to catch pedophiles. The whole point is that back doors are not good, period. If you want to catch pedophiles, that's good, but don't do it while putting in place a massive data collection from people's smartphones or cloud storage, even if its only the Hash. If for an example a hacker breaches your iCloud password, he could upload this type of indecent images and you will have a hard time explaining to the cops that these photos are not yours. This same technology could also be used for different purposes if you substitute child violence with for example images of political pamphlets. Virtually half the world's population lives under some level of totalitarian regime where only a few care about privacy or democracy, so the iPhone should be a fortress as far as personal information is concerned IMO.