You know what really bothers me the most? There's a lack of context.
I can send you off to www.mysite.com/mypage.html and plastered on there can be a blacklisted pic. I can advertize it as a funny pic, you don't know, you wont know until you see it. Now obviously a SITE like that wont stay up for long, but posted on a big site quick enough you can frame many many people who had no intent.
Additionally with sites like 4chan. I'm sure a lot of 16-17 year olds go through that site but they look like they could be 18+, you can't really tell, and there's no way to be sure. If one of the pics posted on there is blacklisted, bam that's a lot of people who though they were looking at an 18 year old and soon are pegged w/ this problem.
The real issue is INTENT, did the offended INTEND to see kiddie pron? 9/10 cases, probably not. The difference is, did you close the page? or save the pic? and no level of government bullshit shy of tapping your pc is going to come close to detecting that.
It's like buying shoes, and then later the police come to your door and arrest you for buying shoes that were stolen. You didn't buy them BECAUSE they were stolen... but the govm't isn't willing to make the distinction.