There is a less generous reason for the Red Army to leave Petsamo in 1940. Building a nickle mine under arctic conditions was technically no simple task with the technology of the 1930's, especially when there was only one road leading to the site from Finland for transporting supplies.About the political goal if Finland had been defeated militarily - why on earth should Stalin have given up politically something that had been conquered militarily with the sacrifices of the Red Army?
probably for the same reason they left Petsamo after the war.
The Western capitalists of the International Nickel Company had not yet built the mine ready for production in 1939, so in 1940 Stalin could quite calmly wait and let them complete it -- especially, if he knew he was going to take it later anyway.
When the mine was already in operation, he had nothing against taking it in 1944.
Hanski