But the crux is that these people are not serious (though I'm sure some are), they're just saying it for the sport of calling the bluff of anonymous people they perceive as trolls or brooding teens.
It's like the scene in Futurama where Hermes is threatening to jump off a building to his death and Bender says, "Do a flip!"
This doesn't make it right. And doesn't absolve them of the legal hot water they'd find themselves in. But I hope these individuals, if charged with a crime, get lawyers who will vigorously defend this point.
Edit: I want to re-state that I emphatically do not hold any opinions on this topic. I'm just trying to share a bit about what these image boards can be like.