Via Jason Calacanis:

The way this works is that stories on digg can be buried (voted down), but unlike positive votes, negative votes don't have names attached to them. This was done in the early days, from what I was told from insiders, so that the staff of digg could kill stories they didn't like and blame it on the will of the community. This kept the digg staff's fingerprints off of things that were killed so the staff of digg could say "we didn't kill it, the community did." Very smart... but now it's coming back to haunt digg. I'd love to see the buried votes on some early anti-digg stories... you can be sure digg will never release that data.

What this does is ensure that Digg will become more and more like an overcrowded Usenet board - the trolls will end up owning it.