It's not so much that it's personally offensive. It's a moderation issue because such comments (a) add noise, (b) undermine trust, (c) are extremely common, and (d) are overwhelmingly imaginary. I say (d) based on personally having studied this data closely for years.

If you look through the past explanations at https://hn.algolia.com/?sort=byDate&dateRange=all&type=comme... you'll find plenty of...past explanations.