That's a bit of a generalization but I can see where you are coming from.
It's important to realize though that China was a different animal as far as the nations involved in the Second World War are concerned.
First of all, China was the only major country in WWII that was divided into spheres of influence and had been long before this war. This really dates back to the Opium and Arrow Wars, but there had been an expansion in this leading up to the Boxer War at the beginning of the 20th century in which seven western counties (Japan, and the United States included) fought both the Boxers and the Qing's Army. The Qing, the last dynastic power in China, collapsed in 1911 and by World War I fighting in that theater was about these spheres of influence and not about any Chinese nationalistic fervor. Japan invaded Shangdong province and took Qingdao (or Tsingtao if you are a beer connoisseur) which was the German sphere of influence in China at that time. The Chinese had hoped that they would be able to win back this province at the Paris Peace Conference, but the Japanese exerted enough influence at the conference to keep the issue off the table. The Chinese protested by refusing to sign the Treaty of Versailles, the only member nation at the peace conference to do so (in fact they didn't even show up at the signing ceremony). It would not be until the Washington Conference, most widely known for its naval aspects, that Japan would give Shangdong back to the fledgling Chinese government.
All the while, as these spheres of influence continued to influence Chinese politics and nationalism, the United States was still pressing for the fiction of an "Open Door" policy in China. So the Japanese expansionism in China did play a roll in the U.S. reaction to Japanese imperialism to a degree.
However, it was more an issue of the balance of power in the entire region that was a primary motivator. Notice that the embargoes of 1940 and 1941 are not related to China at all, but to Annam (French Indochina). China by this time has pretty much been reduced to skirmishing and though there are efforts being made to aid the Nationalist government, it is a drop in the bucket, and the effect is insignificant. (An interesting side note, did you know that Lend-Lease aid to China was the only Lend-Lease aid that was not provided directly to the recipient government instead being controlled by the American aid in theater, like General Joe Stilwell).
Indochina however, creates a whole new problem with the balance of power in the South Pacific. Now we have Japanese expansionism directly threatening Western colonial possessions. The Dutch East Indies, Singapore, Hong Kong, the Philippines are all now in the line of fire of the Japanese. This was a larger issue than that of the Nationalist's struggle for control in China (meanwhile the CCP is slowly growing in the north to become a problem for the Nationalists down the road). While the official American position is one of anti-colonialism, these areas, especially the DEI are still trading partners and as Alfred Mahan would say, these lines of communication must be maintained.
Of course, the Japanese could have chosen to get the materials peacefully.
but the real problem remained the army in china, and the inability of either the Navy or the civilian government to control the army much. the army was paralyzed by what they called “debt to the dead” meaning by that time they had lost 200,000 troops and couldn't pull out with nothing just because the US said so. so the US presented a basically unsolvable problem to the Japanese, pull out of china, which they couldn't do, or shut down the country in two years or less for lack of oil and materials.
its for this reason i have to feel that the US was a party to the drive to war, and that war was the predictable outcome.