The reason, IMO, is that Ahmadinejad is a friend of Russia.
http://www.kansascity.com/451/story/1254226.html
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From CBS-News, July 29, 2006:
Chavez Vows To 'Stand By Iran'
After Oil Talks In Tehran, Venezuelan Leader Called 'Brother' By Ahmedinejad
"Chavez pledged that his country would 'stay by Iran at any time and under any condition,' state television reported. Ahmadinejad said he saw in Chavez a kindred spirit." "'We do not have any limitation in cooperation,' Ahmadinejad was quoted as saying. 'Iran and Venezuela are next to each other and supporters of each other. Chavez is a source of a progressive and revolutionary current in South America and his stance in restricting imperialism is tangible.'":
http://www.cbsnews.com/stories/2006/07/29/world/main1847331.shtml
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President Obama and Venezuela dictator Hugo
Chavez at the 2009 Summit of the Americas in Trinidad
Obama, Chavez shake hands at Americas Summit:
http://www.breitbart.com/article.php?id=D97KK2T00&show_article=1
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From National Public Radio (NPR):
August 29, 2006
"Venezuelan President Hugo Chavez has been visiting countries such as China, Iran and Russia as part of an effort to build a 'strategic alliance' of interests not beholden to the United States. He considers the United States his arch enemy.":
http://www.npr.org/templates/story/story.php?storyId=5729764
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Russia's Medvedev hails "comrade" Obama
Associated Foreign Press (AFP) ^ | April 2, 2009 | Anna Smolchenko
"Russia's Dmitry Medvedev hailed Barack Obama as "my new comrade" Thursday after their first face-to-face talks"
http://www.france24.com/en/20090402-russias-medvedev-hails-comrade-obama
April 1, 2009:
"Obama, Medvedev pledge new era of relations":
http://news.yahoo.com/s/afp/20090401/wl_afp/usrussiadiplomacynuclear_20090401152002