The relevant rules are unclear. Under the "notes’ section for Russian militia we find the “move 1 within Soviet Home Country” clause. Under the “Mobilize the Reserves” special rule we find “All Soviet Militia have a move of 1 once the USSR is at war with a Major Power.”
Now we have two possible interpretations. First Soviet militia gain the ability to move once at War with a major power within their home country. Second Soviet militia have the ability to move within their country from the start and gain the ability to move outside their country once at war with a major power.
I tend towards the second interpretation as it take each rule as written without adding clauses. The first interpretation requires that"…once at war with a major power" be added to the note section and “within Soviet Home country” added to the special rule.
I do not know the “official” ruling however the direct interpretation of the rules as written would be that once at war with a major power Soviet militia have a move of 1 without geographic restrictions.