> "How do they draw a legal distinction between a pervert and an idiot who clicks a link?"
Much of child porn law centers around "intent" -- it's not illegal to see child porn (and be like "oh nasty, don't want that, alt-f4"), it's illegal to intentionally produce, procure, possess, or distribute it.
If the FBI controls the server, they can monitor connections and behavior. Did the user open the site and then immediately leave? Did they scroll around? Did they click images or videos? Did they access multiple pages which are clearly identified as perv material rather than leaked juicy Sony e-mails? Loading the site and then immediately leaving doesn't show intent, but loading the site and then digging around on it does. (There's also the next level -- once the FBI identifies a potential perv-or-idiot and seizes their box, they can check for additional evidence, like whether someone has accumulated a collection of child porn.)
The legal defense concept that would apply is called an "affirmative defense". It basically says yes, you did the thing in question, but explains that there was no criminal intent. Like, yes, I clicked on a link that took me to a website with illegal content, but I was misled, as you can see from my behavior of immediately hitting the "back" button. (Likewise, if you find in your large porn collection that a few images are actually illegal, you can safely delete them or turn them over to police -- the fact that your main collection is legal, and that you acted to get rid of the illegal content, shows that you did not have criminal intent.)
Much of child porn law centers around "intent" -- it's not illegal to see child porn (and be like "oh nasty, don't want that, alt-f4"), it's illegal to intentionally produce, procure, possess, or distribute it.
If the FBI controls the server, they can monitor connections and behavior. Did the user open the site and then immediately leave? Did they scroll around? Did they click images or videos? Did they access multiple pages which are clearly identified as perv material rather than leaked juicy Sony e-mails? Loading the site and then immediately leaving doesn't show intent, but loading the site and then digging around on it does. (There's also the next level -- once the FBI identifies a potential perv-or-idiot and seizes their box, they can check for additional evidence, like whether someone has accumulated a collection of child porn.)
The legal defense concept that would apply is called an "affirmative defense". It basically says yes, you did the thing in question, but explains that there was no criminal intent. Like, yes, I clicked on a link that took me to a website with illegal content, but I was misled, as you can see from my behavior of immediately hitting the "back" button. (Likewise, if you find in your large porn collection that a few images are actually illegal, you can safely delete them or turn them over to police -- the fact that your main collection is legal, and that you acted to get rid of the illegal content, shows that you did not have criminal intent.)