For starters, some of those nations did not enter into the Soviet Union willingly. And I notice that you didn't mention the secession disasters such as Yugoslavia, nor the old Soviet Republics that have seceeded and then gone bad. Secession is not something to be done quickly or lightly, even when it can be justified. And when you do not have the cooperation of all of the parties involved, you invariably wind up with a situation similar to the Civil War (see Chechnya).
The broader question revolves around the scale at which you think a people has the right to seceed. Obviously, you think that a state can seceed from a country and you seem to agree that a county can seceed from a state and join another country. What about a town? Should Niagra Falls, NY be allowed to seceed from New York State and the United States to join Canada? What about a large farm? Should a farmer in Montana have the right to seceed from his town, county, and state and join Canada? I'm trying to understand your limits, if you have any.