samurai007, i just checked online on some french sites....
it seems i was wrong... but unlike american reports, it did say that Greer didn't have a flag, or markers (something like that) to show that it was american, and because it followed the sub for a couple of hours, according to Commander Fraatz, he fealt that they were in danger and didn't realise that he was dealing with americans.... And like you said previously, he fealt that the actions of the greer and the british plane seemed rather hostile, thus the need to react!!!
I apologize, i guess not reading your history books for a couple of years doesn't help!!