The terms CP and pedophilia are being thrown around without much thinking, the same way the general public thinks all who are on torrent sites are pirates and "hackers are baad and steal your CC numbers". This sort of blunt scare mongering demagogy is commonly used to create support from the masses, I am amazed that the HN crowd is also susceptible to it.
Here are the facts as I believe them, please point out the ones you think are wrong:
1) Some of these subreddits may have contained CP, the illegal ones were being actively removed by admins.
2) Calling pictures of 16-year-olds in bikinis CP is not very useful and dilutes the term.
3) What is legal and morally right usually does not overlap 100%. Law may say one thing about CP and people may feel another thing. This is OK. Also different cultures, of course, have different attitudes.
4) In light of (3), to me there's "definitely bad CP" and "tolerable CP". The moral distinction is not clear cut and the law does not differentiate these. The situation is similar to the 55 mph speed limit, people break it everyday (avg speed on I-90 is more like 75mph) but if you do 85 the police will get you. So there's a tolerable zone and to me (2) falls in that category.
5) One has to be very careful with these outcries because they have the effect of ratcheting the law machine ever more tightly. Remember, no politician will put relaxation of sex offense laws on their ticket.
But, please THINK A LITTLE AND RESEARCH before you pick up the pitchfork, e.g. read this (http://www.economist.com/node/14164614).
ADDENDUM: For (2), also consider the widespread use of young girls in advertising and movies, e.g. the Vanity Fair topless photoshoot of Miley Cyrus who was 15 at the time (http://en.wikipedia.org/wiki/Miley_Cyrus#Controversies). This is just one that came to mind instantly, many more examples can easily be given.