Oh look, a nifty illustration.
Suppose that as a starting point, before your vote, both Kerry and Bush have 50,000 votes.
If you vote for Kerry, he takes a 50001-50000 lead, +1.
If you vote for Nader, Badnarik, or other minor candidates, it stays 50000-50000-(some insignificant number). The candidates' net margin over each other is ±0, so compared to a vote for Kerry, this result is +1 favorable to Bush.
If you vote for Bush, he's the one that goes up 50001-50000, for margin of +1 Bush or -1 Kerry. This result is +2 favorable to Bush when compared to a Kerry vote, since he now has a margin of +1 instead of -1.
So using the logic that considers a vote for Nader to be equivalent to a vote for Bush, a vote for Bush is equivalent to 2 votes for Bush.
Q.E.D.