That is because according to US Japan security treaty they do have to defend Japan if Japan is attacked by outside force, and under the 1971 Okinawa Treaty US placed Diaoyu Island under Japan's control. The thing is now US is admitting this is a dispute and as long as no one is attacking anyone there it is neutral in this dispute where it really shouldn't.The US has a legal obligation to defend the Diaoyutai from a Chinese invasion, so don't expect the US to side with China on this, even though Diaoyutai really belongs to Taiwan.
No matter how US choose its side in this. This dispute should never be about Japan and China, in should be a China US dispute.