Because it's such a rare corner case. I've been running my blog for 10+ years and apparently I've never seen a request without the UA header - until a post got

Because it's such a rare corner case. I've been running my blog for 10+ years and apparently I've never seen a request without the UA header - until a post got to the front page of HN a while back and someone like you caused a bunch of "Use of uninitialized value" errors in my logs. It's fixed now.