Considering the amount of visitors on HN, it's possible you've never seen or met anyone matching the described behaviour (in the linked pages). But the people on the receiving end might not want to warn for the thousands of people it doesn't apply to, but rather the ones where it does.
Say it's only 10% of 1000 visitors matching the HN referrer, that's still 100 people against a handful of volunteers. If only 10 of those think it's funny to SWAT them, or send them things they didn't order, all day, every day, that's enough to warrant such a warning.