This. It also has the perverse effect of leaving the defense of "racists" to actual racists (e.g., [1]), thereby leading to guilt by association: "See, only racists are willing to defend this guy, so he's obviously a racist!"
[1]: https://twitter.com/B8GodDante/status/607201146017615872
Many moderates - in this case, moderate supporters of Yarvin, as shown in the comments here - appear to be fearful of getting tarred with the scarlet "R" and facing career threat. The scarlet "S" and "M" are also significant.
In contrast, the "C" is not threatened nowadays, if it ever was. Left-wing extremism is given a pass, while right-wing extremism is always tarred with the "R" brush. People rarely use the "F" word - they just default to "R", which is far more damaging.
(I mean: racist / sexist / misogynist / communist / fascist above.)
And this reminds me of a quote by the not moderate Jim Hightower, "There's nothing in the middle of the road but a yellow stripe and dead armadillos."