Comment by card_zero
They did try to, as well, in Manvers. Can't remember if this was before or after her post, and presumably they weren't really following her instructions, but given the high probability that her post might cause somebody to set a hotel on fire, it seems that she could have been convicted even under US law.
"Bins were set alight and pushed against fire exits"
Some people broke the law, and the poster can be held accountable for successful incitement. It's not the post that broke the law per se.
You can yell fire in a crowded theatre. Just hope you don't cause a panic or you'll be in trouble.