It seems intuitively obvious, but I couldn't tell you why until just now. I think I've finally figured out my answer, with levels upon levels of implications, to those who are angry with Nader voters:
The ends don't justify the means.
I voted for Nader because I wanted him to be president. Voting for anyone else would be against my personal ethics. If you don't vote on principle -- or if you don't vote, on principle -- then you are gaming the system just as corruptly as those who record false votes on behalf of graveyard residents.
As always, there's more I could say about this....
The ends justifying the means is an example of the fallacy called Appeal to Consequences. Arguments made to in support of this fallacy tend to fall under the Fallacies of Distraction, particularly the False Dilemma.