Because it doesn't harm anyone. If censorship laws are all that's preventing genocide, it's not like people are going to go, "Well, we'd love to genocide that other group, but these pesky censorship laws, I guess we'll have to find something else to do..."
Anyway, your argument, whether intentionally or not, is a kind of motte-and-bailey fallacy. You accuse 4chan of allowing "absolutely reprehensible political views". In many circles, that description would include views like "Women shouldn't be allowed to play in men's sports" or "Young children shouldn't be allowed to have sex-change surgery". That's the "bailey". But rather than defend that, you fall back to the "motte" of things like conspiracy to commit murder. In some peoples' views, abortion is murder, should we censor talk encouraging abortion? Of course not; that would be me countering your motte-and-bailey with my own motte-and-bailey.
The fact is, private companies shouldn't be allowed to choose what we can talk about. We DO have people allowed to choose that; they're called legislators, and if you dislike the things people are saying on some website, you should take that up with your legislators, not with the website.
> Because it doesn't harm anyone.
Threats don’t harm anyone physically. Similarly, conspiracy to murder isn’t an actual murder until the murder is carried out. Calling for a genocide isn’t an actual genocide, but it’s hard to see what purpose it serves other than being the first step to enacting a genocide. There are plenty of other examples of speech acts rising to the level of criminality that no ordinary person would consider to be Orwellian.
> If censorship laws are all that's preventing genocide, it's not like people are going to go, "Well, we'd love to genocide that other group, but these pesky censorship laws, I guess we'll have to find something else to do..."
By this logic we shouldn’t have any laws, because people will always find a way to circumvent them.
> You accuse 4chan of allowing "absolutely reprehensible political views"
You are quoting a different user. My chief contention is that the sort of material you can find on /pol/ often rises to the level of incitement and that there isn’t anything wrong with prosecuting people for it. The same logic used to justify the criminalization of threats can be used to justify the criminalization of hate speech. The meta then shifts to inventing new or redefining existing categories of violence, sure, but this is just a slippery slope fallacy which assumes that there will be an endless tolerance for bad faith interpretations of an existing law. Outlawing murder has not led to the definition of murder becoming so expansive as to prohibit the general public from discussing the death penalty, for example.
> the criminalization of threats can be used to justify the criminalization of hate speech
This doesn't make sense to me. The reason threats are only illegal when they are "directed to inciting or producing imminent lawless action and is likely to incite or produce such action"[1] is because of 1A.
Hate speech isn't well defined. As the GP stated, some may say that abortion is murder. To others, it is hateful to suggest they don't own their bodies. Same argument can be made for trans, gays, etc.
Disallowing speech like, "Kill all Christians! They're ruining our nation." doesn't incite people to imminent, lawlessness. There is a massive jump mentally the reader must make to transform that statement into action.