I have no opinions or expertise on the legal merit of any kind of defense. I'm just sharing what happens and what's going though the minds of most of the people in these threads.
You just have to spend some time on 4chan and have common sense.
I won't link it but right now I took a minute to find an example. There's a thread on /b/ where someone claims their mom found their bottle of urine and asked what should they do. Countless people said to basically murder their mother.
So do you think those people are truly encouraging a murder? No. They're going along with the fiction. It's just part of what 4chan does...
On the other hand, if that person did murder their mother, all the people who told them to do so would be co-conspirators.
We're at the stage in society where if someone says "should I shoot up a synagogue" and you say "yes", you should be prosecuted, because otherwise it's going to keep happening.
Trolling the trolls is just what you do on /b/