Because HN has a reputation and this influences that reputation. The same way an open relay would harm your MTA reputation or an open redirect harms your domain reputation.

So if HN were to think: "gee, we don't want to delete this content, but we also see how it breaks the rules", they could opt to put some noindex flags for the robots to consume which is a very cheap way to try to do something that reflects their own rules.

It is of course possible that this is no reputation effect at all, and there is also no interest on a human level in all of this. In such a case, nothing happens.