What's the link? When people bring up communism, other people describe what happened in communist systems. How is that guilt by association? What aww_dang means

What's the link? When people bring up communism, other people describe what happened in communist systems. How is that guilt by association? What aww_dang means by that is that someone will write something libertarian or conservative, and the HN comments will fill up with replies of the form "did you know that on the same website is a different article by a different author that says something libertarian?!" which is a pointless sort of post. It is a form of signalling to other leftists saying "attack!", and against the ethos of the site as it's not intellectually curious.