Stalin also allowed German destroyers to sail around northern Russia to attack Allied shipping in the Pacific.
I personally believe that if the US/UK can drop land units off into Archangel/Karelia the game is essentially broken. By all means allow them to send IPC aid to Russia, but historically Stalin did not want to share the glory of victory in Russia with foreign troops.
There must be a handicap to doing this; at the very least the turn it takes for Russia to convert this income into units and get them into action.
In a sense, the eastern and western fronts are depicted in different scales, certainly in regard to the numbers of units represented by infantry and Armoured pieces. If it were accurate, the IPC income and starting units for Russia relative to the other Allies would be significantly bigger, perhaps 50%+. That’s why introducing US/UK units into Russia can be so imbalancing.