Because there wasn't really anything to gain or lose by interfering or choosing not to interfere by European nations. The US was pretty much irrelevant at that point in time, so there isn't much it could have done. That said, to compare Japan, a resource hungry nation, to a global superpower, and to compare two completely different situations from two completely different time periods is, to be kind, silly.
Why'd those ******* Vikings kill Native Americans?! Why didn't the Brits stop them!?!!!
Yeah, no.
@Grega: A blind eye wasn't turned. Europe appeased Hitler, hoping that he'd leave them all alone once he had his fill of conquest. Why? Because they were still recovering from WW1. The US didn't so much turn a blind eye as it did do business with all parties involved. Everyone wants the US to be neutral until they get ****ed in the ass. Then it's time to choose sides. Luckily, for all parties, it was in our best interest to preserve the Alliance.