The US eagle has its wings going up, not straight sideways. And the head is pointed to the eagle's right, not the left. (This is also the difference between the reichsadler, which is a more generic symbol of Germany, and the parteiadler, which is a Nazi symbol: the riechsadler's head points to the eagle's right, while the parteiadler's head points left.)
I think the shape is quite dissimilar to either (e.g. the Nazi one has legs and no tail). I wanted to agree with you on the head thing, but it seems that Nazis used both directions... I'm not sure I'd call the Reichsadler a "generic symbol of Germany" if it was used during the Third Reich...
The reichsadler goes back a long way and has been used in other times. That particular style appears to be used in the third reich, but the distinction remains: the right-pointing head signifies "Germany," and the left-pointing head signifies "NSDAP."
In any case, that shirt design screamed "Nazi!" to me the moment I saw it. I didn't set out looking for Nazi symbols, either, I was just browsing to see if there was anything that might be considered offensive. I was rather surprised that they would be so brazen. I can understand if you don't think it's sufficient, but I do.
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