And by letting all USSR member states the option to break away, but almost none of the US states, you also give the US another big gameplay advantage, unbalancing the game even more.
Well according Article 72 of the Soviet Constitution , the option for those Republics to break away was allowed.
Article 72. Each Union Republic shall retain the right freely to secede from the USSR.
http://www.departments.bucknell.edu/russian/const/77cons03.html
A US Supreme Court ruling in 1869 says that US Constitution did not permit states to suceed.
http://www.law.cornell.edu/supct/html/historics/USSC_CR_0074_0700_ZO.html
Less legal speak on the Wiki
http://en.wikipedia.org/wiki/Texas_v._White
As far as the game goes, having colonies may be more beneficial especially having colonies close to ones homeland. The Devs havent gotten into all the specifics of what advantages of having colonies are, they only suggested they could be used as possible places to create tensions.
IMO The USSR has the advantage on colonies. I see WW3 starting if the US players messes around with any of the SU colonies in any way and gets caught while the SU could probally steal Guam w/o starting war. Messing with PR and the Canal Zone might be tricky but if they get a good hold in Cuba then those Colonies might be doable as well. Thats not counting what benefits of having colonies might bring that the Devs havent already mentioned.