So? The Great Powers had decided on a policy of appeasement against Nazi Germany. What exactly would you have proposed the USSR do? They signed the Molotov-Ribbentrop Pact prior to the war for a reason.

Without the Eastern Front, Europe was lost. Hitler only launched Operation Barbarossa because he thought the Western Front was all but won. Continental Europe was under German control and the UBoats were locking down most of the Atlantic, meanwhile imports of Russian materials was sustaining the German war economy (similarly, imports of American materials was sustaining Japan's war in China and the Pacific)... Of course, it turns out that dividing your forces and taking on Russia in the winter aren't the best ideas, but at the time Germany wanted energy independence and the Caucasus was seen as an easier target than the Middle East (which at the time out produced Romania but wasn't yet the oil superpower it is today).