OT hijack: This can lead to news from popular sources being submitted because they are known to generate lots of votes. This leads to lots of people trying to be first to submit which leads to them getting lots of "upvotes", which further contributes to that source being known for upvotes. This leads to certain sources now always appearing regardless of the merit of the actual articles.
If I know that every codinghorror post gets to front page, I know that if I spot a new coding horror post I should submit to HN to get the karma. In an effort to be first I shouldn't check if it's previously been submitted, I should just submit.
Therefore I will submit. If it turns out I'm not the first I'm just upvoting the article, if it turns out I am first I "win" a lot of karma.
There's no way to differentiate between a genuinely interesting coding horror post that is getting submissions from people because it's a good post (of which there are many) and between a coding horror post that is getting submissions because people expect them to get submissions.
After a while, even low quality sites that are known to have people who post them for this effect can get trapped into this cycle.
This isn't an effect of spammers, it's an effect of a community who know how each other behave.
If you can't differentiate between this and more "genuine" activity then you cannot dis-incentivise the activity you wish to prevent.
Thanks - that makes a lot more sense than some bot working around a spam filter. Seems like an obvious loophole as well.