But DiRosa’s Facebook comment is likely protected by the First Amendment, unless there are more facts that aren’t being reported. It isn’t punishable incitement of crime — to fit within the incitement exception, speech must be intended to and likely to produce imminent unlawful conduct, as opposed to just being “advocacy of illegal action at some indefinite future time.” See Hess v. Indiana (1973) and Brandenburg v. Ohio (1969). And this post, even if it is intended as advocacy of murder, is indeed advocacy of murder at some indefinite future time (as opposed to in the coming hours).
Nor is it likely to be a punishable threat. It is far from clear that it is indeed a statement of what the speaker would do (“I will kill police officers”), which would be subject to the threats test, as opposed to a statement of advocacy (“you should kill police officers”), which would be subject to the incitement test. But even it is a statement of intention, such general statements, with no specificity of victim, place, or time, are not punishable threats.
The police have ample authority to investigate people who say such things, and see whether such speakers are actually planning something beyond speech. But prosecuting a person based on such a statement is, I think, forbidden by the First Amendment.