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.
https://en.wikipedia.org/wiki/Coat_of_arms_of_Germany#Nazi_G...