Because “seeing” a crime and doing nothing to stop/prevent it, or even worse, “enjoying” it, is being an accessory after the fact.
No non-evil person could see child pornography and not want to alert authorities about it. If any one were to “see” child porn on a regular basis, they are assisting the production of it by being a user, whether they pay money or not or download or not.
If there were some anonymous way to rat out a locating of online kiddie porn... I believe every single soul here would be all ears, no matter what they thought about the legalities of this present situation.