Most such 'western' Laws are extremely intrusive and offensively stupid.
As with most drugs, guns, many other 'illicit' items (i.e. those not understood or approved of by the Tribal Witchdoctors of the society), simple possession is enough - even if you didn't know the stuff was there.

Neither the Law-makers (the Pollies), the Courts & law enforcement Nor the average members of the public understand *anything* about the Internet, P2P, Usenet, Online Apps etc. etc.

In a long tradition of entrenched ignorance and bigotry that goes back to before The Pious & Insane Torturers of the Christian Church's 'Inquisition' thru the Witch-hunts of Salem, Nazism, the McCarthy horrors in the US to the Current 'War on Drugs', 'Tough on Crime' and 'War on Terror' *nonsense* that's used to 'justify' appalling legislation destroying our privacy & basic liberties, the approach to 'The Internet and Child Porn' is as fundamentally stupid and unjust:

If you're reported/investigated and found 'in possession' you have as much chance of 'Justice' as young Nguyen Tuong Van (Arrested: Waiting in Singapore Airport *transit* on his last leg home - he hadn't even entered the barbarous place - Jailed & Hanged on a ludicrous pretext: Importing & Trafficking! http://tinyurl.com/744gc) or young Australian 'Innocent abroad', Schappel Corby (http://tinyurl.com/b5kcr) -arrested arriving from Oz at Bali airport , when customs 'found' 4kg of cannabis leaf in her bodyboard bag - Faced Death, got 20 years in an Indonesian Hellhole..If you don't know, taking Cannabis to Bali is like sand to the sahara - it's about 100 times cheaper there than *anywhere* in Civilisation! Utterly ridiculous and entirely barbaric & corrupt -They didn't even fingerprint the plastic it was wrapped in! The going rate is about A$100,000 for the charges to 'go away' or even just a Sentence reduction...-If interested, read this moron, too: http://tinyurl.com/af8aw -'The Law must prevail' no matter what the outcome!]

Precisely the same 'principle' applies to Child porn: if found 'in possession' you have *NO* chance, no excuse - which means *any* computer user/ISP could face serious criminal charges at any time.

It is bad Law and it should be changed.
But that's the way it is.