Deadbunny, you may be exactly right and such comments are one of the reasons I wanted to post the idea for discussion.
While I mull it over, let me say that while it is true that Japan could easily get three IPCs worth of territory from USSR, they would have to spend serious resources to take those three territories if Russia defends them with some of those 18 infantry. If they utilized some of those same resources to do more damage against China and/or elsewhere, the NO would still give them 3 IPC bonus so the net result would possibly be better for the Japanese (though perhaps not for the Axis as a whole). Meanwhile, some of the boost Japan gets in this scenario would be countered by the boost USSR is getting for not attacking. I agree that Russia doesn’t have as much incentive to attack as Japan, but the 18 infantry can be a tempting offensive force, esp. if the Japanese player moves forces southward into China. The NO would give them an additional disincentive to attack early in the game (something I like) and a counter-balancing reward to make up for the advantage given the Japanese player if they decide not to attack USSR.
The point of the NO is not to discourage a Japanese attack on the USSR, which I want to remain a viable option, but rather to make a path that does not involve attacking the USSR equally interesting and viable. My particular house rule might not fit this bill if it still seems like the “best” option is an attack on the USSR with or without the NO, which is what I take deadbunny’s critique to say. Do others agree? If so, what might make a Japanese avoidance of war with USSR equally viable to an attack on the USSR?