From the opinion in Hustler v. Falwell:
"At the heart of the First Amendment is the recognition of the fundamental importance of the free flow of ideas and opinions on matters of public interest and concern. "The freedom to speak one's mind is not only an aspect of individual liberty -- and thus a good unto itself -- but also is essential to the common quest for truth and the vitality of society as a whole." [CITATION OMITTED].We have therefore been particularly vigilant to ensure that individual expressions of ideas remain free from governmentally imposed sanctions. The First Amendment recognizes no such thing as a "false" idea. As Justice Holmes wrote, "When men have realized that time has upset many fighting faiths, they may come to believe even more than they believe the very foundations of their own conduct that the ultimate good desired is better reached by free trade in ideas -- that the best test of truth is the power of the thought to get itself accepted in the competition of the market . . . ." [CITATION OMITTED]
....Generally speaking the law does not regard the intent to inflict emotional distress as one which should receive much solicitude, and it is quite understandable that most if not all jurisdictions have chosen to make it civilly culpable where the conduct in question is sufficiently "outrageous." But in the world of debate about public affairs, many things done with motives that are less than admirable are protected by the First Amendment."....
"Debate on public issues will not be uninhibited if the speaker must run the risk that it will be proved in court that he spoke out of hatred; even if he did speak out of hatred, utterances honestly believed contribute to the free interchange of ideas and the ascertainment of truth."[CITATION OMITTED]. Thus while such a bad motive may be deemed controlling for purposes of tort liability in other areas of the law, we think the First Amendment prohibits such a result in the area of public debate about public figures."
In Phelps,the court further explained that where the speech relates to a matter of public, rather than purely private, concern, the First Amendment restrains the imposition of civil liability.
In short, the controlling factor is not whether the case is criminal or civil or whether the elements of the tort were "subjective"... its whether the speech at issue is protected or not.
Finally, turning back to your original example, the "prankster" couldn't be held liable, even if the poster was deemed purely private speech because it would not be possible to prove that the attack on Group A was foreseeable to a prankster who didn't know Group A was in the building.
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