Without the 'back link', it cannot redirect you to your original search. Your 'from location' is google.com/sorry, so, once you pass the test, it sends you back to google.com/sorry instead of back to your previous activity which triggered the Captcha.
If you're coming from an HTTPS page (which I think HN forces) to an HTTP page, the referer is not sent. It is only sent for HTTP->HTTP and HTTPS->HTTPS (even cross-domain, despite the info that leaks out)
