Because they made the platform. In some light, it’s the same logic that allows businesses to refuse service, or homeowners to defend their property.
The difference is that on HN the political content (so far) remains contained to a few articles every now and then, plus you can still read 'flagged' comments. Even if I have a strong political opinion I prefer HN in this curated way.
On Reddit on the other hand every major subreddit gets politicized eventually (/r/pics, /r/news), on-topic discussions being derailed to shout about Trump, calling people "Nazis" and the mods will rather ban or delete the posts that call the activists out or which cite 'inconvenient' facts. Complete threads get nuked in the most intransparent ways which can be seen using third-party tools like https://snew.notabug.io Inside the major subreddit only one side (the 'Left' side) of the political spectrum is allowed to voice their opinions even if those were off-topic in the first place.
Why should platform owners decide what's right and what's wrong? If something is legal then why should we censor it just because it may seem hateful to someone, or somebody feels offended? So what if a post offends somebody? They can deal with it like an adult. Ignore the trolls, and discuss if there is a discussion, offending or not.
If people want a safe private place with no intruders, then they can create a closed/invite-only group and moderate it themselves.