I am not sure, but I guess that the Designer have calculated this swing in some way when he balanced the game, so this issue is taken care of as far as playability is concerned, I hope. But if you got problems when the Conquer get 3 IPC from a territory, what do you think about the 5 IPC NO bonus on top of that again ? I guess you don’t love it. Stalin, the original owner, could only receive 2 IPC from Leningrad, but Hitler is able to squize 7 IPC out of it every turn, and that is even after the territory was bombed, scorched, burned down and the population killed. So I figure the rules, and the NO rule in particular, are not up to exactly what happened in the real war. And if you ask me, I guess it was designed this way to make for a faster game. As simple as that.
But, since we are in the house rule forum, after all.
Russia could get a Scorched Earth national Advantage rule. Every time you capture a Russian territory, put a damage token over the IPC income number. This territory was scorched and nobody get income from it now. You need to pay 1 IPC to remove the damage token during the Repair phase in your next Turn. Now, the loot is at least delayed one turn. But it may effect the balance.
I don’t think this rule should go for France too. France did not blow up the Factories or burn down the corn fields when the Huns come. The GDP was halfed yes, Hitler got less than 50 % of the money France was making before the war, but in this A&A map that is taken care off. In this game France got a ridiculous low IPC value compared to the real France, so lets keep that as it is.