It may certainly have frustrated the Chinese at the time, but there is an awful lot of good reasons for an American ship to not open fire (and officially start a war) upon Japanese ships while on a mission to safely evacuate their own people still in a city being overrun by forces that are officially neutral, but would become hostile if they were to open fire.
Huangpu RiverStanding on the sidelinesAug.13 1937
Battle of ShanghaiShanghaiLook, the Chinese are fighting the Japanese!Nimitz?KMT forcesUSS Augusta came to evacuate the first wave of US civilians. She arrived in Shanghai under the pressure of a typhoon, and was immediately bombarded by Chinese air forces. Thus, she hung three huge US flags on her turrets as IFF. An unknown shell hit her on Aug.20, resulting in 1 death.USS Augusta
She was really there =_=|||