It's deliberate. I believe pg said it's to stop people effectively giving themselves the top comment on a link. Which is exactly what the submitter is trying to do here. Of course, he's the author of the link so it's fair enough. I guess pg would want him to add his comment like everyone else; or put the comment on the webpage itself; or put up with a unlinkified url.
Personally, I think it's a bit of overkill for a specific problem (and I don't know if it actually occurred in practice, or it was only predicted that it would occur). Maybe the HN community can be trusted to not abuse linkified links? OTOH, it's an extremely minor niggle, and I agree that pg's time is better spent on preventing spam and keeping the community civil etc. That's way more important.
Meta-discussion like this is frowned upon, but I wanted to answer your question and also offer a solution: I solve this problem with a customable dictionary lookup addon for firefox, by adding a rule that just opens the highlighted string directly. I come across unlinkified URLs quite often.
