I noted this too. All the other expected nations were available but not Russia. However in my case I chose the option upon original invasion of the USSR to create puppet states. I was then given the option to create the state (I can't remember but I don't think at any dissent hit) at the time the provinces were taken, BUT if not creating at that time it cost 4% dissent. The reason I mention this is that I'm sure I was given a one off chance to liberate Russia, but I chose not to at the time as I was still fighting the Soviets and thought I would do so later, but was never offered another chance. I now have a rather big territory to garrison. Mind you, I can (although it takes ages), give what would be the Russian provinces to my new puppet countries Belarus, Ukraine and the Baltic States if I wanted. I just don't have the time and inclination to do so.