Because according to Harrison Hickman, John Edwards Advisor:
“The business I’m in is a business any fool can get into, and a lot can happen. I’m sure there was a poll like that,” the folksy Hickman told jurors when first asked about a poll showing the race tied. “I kept up with every poll that was done, including our own, and there may have been a few that showed them a tie, but… that’s not really what my analysis is. Campaigns are about trajectory, and… there could have been a point at which it was a tie in the sense that we were coming down, and Obama was going up, and Clinton was going up.”
John Gerston has a very interesting article in Politico today;
http://www.politico.com/blogs/under-the-radar/2012/10/a-pollster-under-oath-137100.html
So Why do they do this?
It’s primarily about fund-raising. A Candidate’s bills keep rolling in long after the chances of winning are gone… if for some reason they’re able to cover their bills they can then send those donations to other candidates of their choosing, giving them leverage to gain employment, influence or party position etc. (Buy favors) In Edwards case, pay for his mistress.
“Under oath, Hickman admitted that in the final weeks of Edwards’s 2008 bid, Hickman cherry-picked public polls to make the candidate seem viable, promoted surveys that Hickman considered unreliable, and sent e-mails to campaign aides, Edwards supporters and reporters which argued that the former senator was still in the hunt —even though Hickman had already told Edwards privately that he had no real chance of winning the Democratic nomination.”
“They were pounding on me for positive information. You know, where is some good news we can share with people? We were monitoring all these polls and I was sending the ones that were most favorable because [campaign aides] wanted to share them with reporters,” Hickman testified on May 14 at the trial in Greensboro, N.C. “We were not finding very much good news and I was trying to give them what I could find.”
Hickman testified that when circulating the polls, he didn’t much care if they were accurate. “I didn’t necessarily take any of these as for—as you would say, for the truth of the matter. I took them more as something that could be used as propaganda for the campaign,” the veteran pollster said.
So Why do we trust the polls? A cynic listens to all things and trusts nothing.
I recommend reading the full article.