After all it's not like anybody reads the guidelines linked above anyway before submitting a link, especially not if they're old hands (and most likely not even if they're not). Maybe a message could be added to the submission process that said: "You've used a URL shortener, bang, you're dead", but that's pretty obviously the reason why. After all, all your other links will get through just fine.
My point is: there are apparently a bunch of rules, and I don't have the sense that I understand them. Worse, I have the sense that asking about the rules is frowned upon. I find this a little chilling, and it seems to fly in the face of the quasi-libertarian spirit I generally find on HN.
Right now, there are spammers repeatedly submitting shortened URLs to their WOW Gold sites and not checking to see if they're killed. That actually stops a good fraction of it, because frankly, they're not very smart people.
If you told them exactly why their current tactic is not working, they'd immediately move on to the next one and the next one until they find one that works.
But this is probably related. I think the anti-spam measures are probably going too far. I'm not a spammer. No site I've ever attempted to submit has been been a spam site, an ad site, or anything else self-serving. Yet my most recent attempts to submit have been stomped on, accusing me of "wasting my time" by attempting to spam HN.
Here's the most recent article I tried to post -- you judge whether I'm submitting spam sites: http://www.w3matter.com/journal/2010/8/29/startups-old-peopl...
All of which are useful. Url shorteners are ok for twitter or IRC elsewhere they're just unnecessary complications.