The Japanese were well within their rights not to declare war on the Soviets since it didn't violate the terms of the Tripartite Pact.
The conditions for military assistance laid out in the Pact was in article 3 which states:
ARTICLE 3. Japan, Germany, and Italy agree to cooperate in their efforts on aforesaid lines. They further undertake to assist one another with all political, economic and military means if one of the Contracting Powers is attacked by a Power at present not involved in the European War or in the Japanese-Chinese conflict.
So Japan only was bound to assist if a currently uninvolved country attacked Germany, not if Germany attacked them. Germany could have done the same with the Japanese attack on the United States which would have put Roosevelt in a real bind since even he knew that he would not be able to call on Congress to declare war on Germany even after the initiation of war by Japan. This is the reason he only called for a declaration of war on Japan in his famous speech. Hitler took care of the rest for him.
Considering the open belligerence the U.S has displayed toward Germany and the actions Roosevelt has taken [lend-lease, destroyers for bases, dividing the Atlantic into defense zones, convoy escort for the Brits, tracking U-boats and sending their positions to the royal Navy, depth charging said U-boats], which under then international law would make the U.S a co-belligerent, Hitler’s decision to declare war on the U.S when he did is not as incomprehensible as it once was.
This pact is tomorrow's headline. It will be interesting to see what the wise guys of 1941 saw as its implications for the future of the war, as compared to our hindsight.