Also worth noting that an actual incitement of violence is expressly illegal under U.S. law, and the ChristChurch shooter's manifesto et.al. is probably already illegal, without requiring a new speech policeYour understanding of the law is wrong. Incitement is assessed (in federal law) by the standard set in Brandenburg v. Ohio which requires both the intention to produce imminent lawless action and the likelihood that it will be produced.
https://www.law.cornell.edu/wex/brandenburg_test
Let us consider a document whose entire text is "Reader, please commit a crime as soon as humanly possible." It is targeted (to the reader), specifically encourages lawless behavior, and has a sense of immediacy.
But does it produce imminent lawlessness? If I say those words to you in person, the time between my utterance and your hearing of it is negligible. If you find me sufficiently inspiring or intimidating you may be moved to act upon my exhortation. But when you read a document, it might have been written 5 minutes ago or 5000 years ago. It may have been produced near to you or on the other side of the world. You may know and care who the writer is, or have no clue. Conversely, as the writer of a document my ability to predict when and where it will be read, and by whom, is similarly limited. Thus, a document by itself lacks temporal, spatial, or social proximity compared to an interpersonal interaction - making imminence virtually impossible to prove.
Likelihood of producing action is very subjective (and thereby also hard to prove). In this case I have offered no incentives or specificity to my exhortations, so you probably feel little motivation to select and carry out a crime and put yourself in legal jeopardy. If you were to cite this document as an exculpatory factor few people would take you seriously. Even if I offered an elaborate rationale and specific directions for committing a crime, when they're in documentary form I don't really know who will read it and have even less knowledge of how readers will react to it; it's very hard to say how anyone could assert a definite probability of 0.5 or greater that the crime will be carried out within any given period.
tl;dr it's almost impossible to prove the US standard of incitement outside of a very narrow range of circumstances, so making the 'incitement is already illegal' argument is the legal equivalent of sweeping dirt underneath a rug and then forgetting about it.