It makes no sense to
you, but for
years, users on certain subreddits were exposed to users and their views from other subreddits thanks to brigading.
Likewise, Reddit and Facebook both have to deal with being judged by all the forums they "host." You may not think it makes sense to criticize Reddit as a whole for hosting white supremacist forums and violent misogyny forums, but the rest of society seems to think that one bad apple forum spoils the entire Reddit and Facebook barrel.
If you're looking for a comparison, I don't have to read any thread I don't want to on HN, just as I don't have to join any subreddit I don't want to on Reddit.
But I know they're there, and that affects my perception of what HN or Reddit or Facebook is.
If it was just about hiding content, we could argue that HN shouldn't censor anything, but if we let people tag threads and comments with labels like "white supremacy" or "SJW raving," then everyone can filter what they don't want to see, so what's the problem?