In addition to the points already raised by @TomAu and @DevSolar...
The Pacific lend-lease route skirted the problem by officially being handled by the Soviets. Supervision and routing was handled by the Soviets. Cargo was loaded into Soviet flagged ships, many US ships were handed over to the Soviets. Since ships on the route might be inspected by the Japanese they did not include war materials. The Pacific route handled things like food, trucks, and locomotives which were allowed. War material instead went via the Arctic Convoys and the Persian Corridor.
Attacking the US/Soviet lend lease would have meant sinking Soviet ships killing Soviet sailors and loss of goods purchased by the Soviets. Since Japan was not at war with the Soviets, and was anxious to not be at war with them, there was little justification to attack their ships.
There's an insinuation in the question that Japan could simply attack the Soviets, and there was nothing the Soviets could do about it. The Japanese had already learned this was not so.
Since 1931 the Japanese had been testing the Soviets in a series of escalating border skirmishes. In the summer of 1939 this escalated to a major battle, the largely forgotten Battles of Khalkhin Gol. Then Corps commander Georgy Zhukov was called in to deal with the Japanese probing attacks once and for all and show what the Soviet army was capable of when it got its act together. Zhukov used massed armor to achieve a double envelopment to decisively end the battle.
After this humiliating defeat, the Japanese attitude towards expansion switched from looking north at Siberia (Hokushin-ron) to looking south at southeast Asia and Pacific islands (Nanshin-ron). The Japanese had already suffered a defeat the hands of the USSR, and now had no motive to pick a second fight by interfering with their shipping.