There is always a tension between security, privacy, and convenience. With how the Internet works, there isn’t really a way - with current technology - of reliably catching content like that without violating everyone’s privacy.

Of course, there is also a lack of trust here (and there should be given the leaks about mass surveillance) that the ‘stop child porn powers’ would only be used for that and not simply used for whatever the powers that be wish to do with them.