As to your other statement, you seem to be under the impression that laws or rules should exist as a formal system independent of actual empirical reality; in other words, they should be the laws of the world you think we should have (e.g., one where all ideologies are morally equivalent) rather than the world we actually do have. However, jurisprudence (the theory of laws and legal systems) usually sees most laws as normative regulations within an actual living society, as determined by empirical observation (although some laws may be "universal").
So, while no one wants white genocide, some do actually want black genocide, and, moreover, such genocides or similarly horrific crimes have actually occurred. Not only have they occurred in the distant past, but their effects still exist with us today. This is why, while white and black may be equivalent in a tabula rasa world, they are not equivalent in our actual world, and that is the world laws try to address. Therefore, there is no equivalence (as measured in our actual society), between "I want a white genocide" and "I want a black genocide".
Likewise, whether a specific reader is "in on the joke" or not doesn't matter, as we're not talking about personal, subjective offense. That black genocide and white genocide (when mentioned in statements) are completely inequivalent in our actual society is an objective, empirical fact. One of them dehumanizes and therefore causes actual harm, while the other does not, and so the laws address the one and not the other. As usual, this is not universal. If, for example, the legal system determines that statements made by a black preacher encourage and promote actual harm, as measured factually, they may well (depending on the legal system) sanction that preacher.