Taken literally it's accusing someone of a specific depraved act, but it's also clearly a term of abuse. My guess (not a lawyer!) is that once a term becomes more associated with abuse the more you're protected.
Hustler basically called Jerry Falwell a motherf!cker but attributed to him a specific act, which they highlighted was satire and not to be taken seriously. Hustler lost in a jury trial and also on an appeal to the 4th circuit. The Supreme Court eventually ruled in Hustler's favor [0]. This is dramatized in the movie The People vs Larry Flint.
[0] https://en.wikipedia.org/wiki/Hustler_Magazine_v._Falwell
[1] https://www.metacritic.com/movie/the-people-vs-larry-flynt/