Considering the two were allies, after December 7th 1941 why did the United States have to conduct the Doolittle Raid from the Pacific when the landmass that is the Soviet Union was within range.
The soviet union wasn't at war with Japan until 1945 so they couldn't base any american troops attacking Japan if they wanted to without declaring war. They even 'detained' the crew of any bombers that landed in the USSR after missions against Japan. This allowed them to copy the b-29 to create the tu-4 (Source:http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?id=1852)
Stalin's mistrust of Britain and the US is well documented, for example he refused to believe the warning that Hitler intended to break the non-aggression pact. He thought it was a Churchillian ploy to cause Russia to mobilise and precipitate war. (Stalingrad Anthony Beevor p5). His fears were reaffirmed when the allies invaded North Africa rather than Europe despite his pleas for them to open an effective second front. Although he was willing to accept material assistance he was very far from allowing the US to set up military installations within Russia.
This was actually just asked a couple of days ago: http://www.reddit.com/r/AskHistorians/comments/388s1e/why_didnt_the_allies_bomb_mainland_japan_from/
Anyway, the short of it is that while the US and USSR may have been allies, the USSR was not at war with Japan before 1945. It had signed a non-aggression pact with Japan in 1941.