> "American" on the other hand is purely a citizenship ...

This may be how you perceive or feel about it, and of course you're not alone, but many other Americans feel differently. Those of us with Colonial ancestors maintained much the same culture and mores for generations; it's evident in the manners and the literature; it's something distinct that we certainly feel as close to an ethnicity. Granted, we comprise multiple European heritages, but those heritages did not define any of us after a few generations. The concept I am trying to outline her is also a very old one: e.g., first Speaker of the House Frederick Muhlenberg, referring to some of his own constituents, said, "The faster the Germans become Americans, the better it will be."

If German is an ethnicity, I don't see why the US, which is older than the German Confederation (let alone the subsequent countries that have existed since then on that same land) has a distinct culture and set of shared values, cannot be.