The reason it is being presented as a vote for a third party is a vote for candidate X is simply this:

• The majority of votes (greater than 60%) will go to one of the two primary political parties
• Anything below 20% to a party will not be in the running to compete
• To vote in a party that will receive less than 20% is negating the vote to one of the two major factions leaving the percentages to be determined by the remaining majority parties
• By not choosing a party in the two major spots, it is presented as taking possible potential votes from the other
  Your vote isn't actually negated, as it does count, however if your candidate can't pull into an area of greater than 20-25%, they are considered a spoiler for one of the other two parties. An example where this actually seemed to make an impact (before backing out of the race) was with Ross Perot in the 1992 election. Your argument will be shut down due to the past examples of this coming to fruition.