I think it can become illegal in the US if you are encouraging people to do criminal acts imminently. That's arguably incitement, but I'm not a lawyer, and this isn't legal advice.
It has to be both imminent and likely - if you saw somebody posting something like that, would you think they were actually going to do it? Nah, it'd blend in with all the other larping and shitposts.
"It" did actually happen, shortly after the post, and was therefore both imminent and likely.
This was also the second such incident within just a few weeks. Knowledge about Christchurch should maybe have informed these "shitposters'". Can't really claim it's all just a joke after people have died.
The jokesterism is in response to a joke world. There are innumerable numbers of murders each year but somehow a spree killing is enough to slay irony? 9/11 wasn't enough to slay irony. The reaction to it excaberbated the ironic mode of moderm culture.
Pre, probably based on a hypothetical reasonable person in the defendant's position. For example:
A provocateur is giving a speech to a crowd against someone or something. The crowd is agitated, and armed. The provocateur commands the crowd to go physically attack the target of their rage. A reasonable person would think that's likely to start a riot.
Someone posts on a message board filled with graphic descriptions of violence the posters purportedly intend intend to commit. The poster says they intend to kill [racial slurs] for the good of white people, start a race war, etc.... Another forum member, knowing that only one or two of the tens of thousands of posts of that nature has been connected to actual acts of violence replies "do it!". A reasonable person would not expect the poster actually planned to commit violence, or that their comment would change the outcome.
https://en.wikipedia.org/wiki/Imminent_lawless_action