There's a lot of questionable things that decent people do, in good faith, because they consider it normal and OK. If you tell people "actually, the convention here on that is something different", then I think most will respect that.

One way this doesn't work is if there's a lack of trust. For example, if an employer claims it values X, but actually behaves like Y, employees are less likely to do X, and also less likely to trust or respect the company on anything else.

Another way the HN example doesn't work as well is if the person has strong motivation otherwise. For example, if their boss told them to post a fake job on HN, and they really don't want to come back and say they can't because they just saw a new rule. But a lot of other times, the person doing the posting has more autonomy, or a more decent work environment.

A lot of guessing here, but I think stating a convention would help significantly.