The logic is to prevent sensational titles. By forcing the title to be the page's heading, it limits posters ability to target the hn audience with a snappy, BuzzFeed-like, heading.
I very much prefer a "snappy, BuzzFeed-like" heading that tells me what the article is about over a meaningless phrase without context.
I think we all understand that. I think we also all understand that there are times when blind adherence to that rule is bloody stupid.