I don't see anything wrong with that considering the limited and volunteer-based resources they have. Youtube-style content ID pre-censoring built into everything shouldn't be a goal we're striving for.
So it ran as a honeypot? I understand that 8chan wasn't hosted anonymously, the owners are known. If they provided a safe haven to child pornography, I assume law enforcement would put an end to that very quickly, especially with obvious and easy ways to apprehend the owners. They've been very active and successful in bringing down hidden services, it's not plausible that they looked the other way for a clearnet site.
I don't think so. What seems more likely to me is that one or more of the following is the case:
1) The "fast-flux" nature of imageboards makes it difficult for law enforcement to effectively respond to illegal content hosted on them.
2) Deleting content upon reports places board administrators technically within the boundaries of the law.
3) The arm's-length separation between the owners of 8ch, the users who operate boards on the site, and the users who post content on the site allows the owners to claim a lack of responsibility for that content.
Only when the platform embraces it does that statement make sense, which apparently 8chan did not if I understand you correctly.
Unless you aren't attempting to imply anything about them being at fault. Although, as long as they are making that good faith effort to take down CP, they can't be taken down by the US govt at least.