At a core, it's because of the same sort of logic as fascists - they'll use dogwhistles and stuff that, on the surface, can imply plausible deniability, because that legitimizes racist discourse in the eyes of the general population. But if you call them out on it, the gig is up, so therefore it's unacceptable that they get called out on it.

It's why they try to criminalize any discussion of systemic racism, and it's also why they want to make it so calling someone a racist is considered uncivil and insulting, even though it's literally just a descriptor for someone's behavior.

No, motherfucker, you don't get away with it - me calling you motherfucker? That's an insult. But when I call you racist? Well, that's just me describing your behavior.