Logically the tax should only be for Germans living in Germany. You should check if there is a double tax treaty between the US and Germany.
There is surely a DTT between the US and Germany. https://taxsummaries.pwc.com/Germany/Individual/Foreign-tax-relief-and-tax-treaties https://www.germany.info/us-en/service/09-Taxes/taxes-income/913924
I misunderstood what you meant about "double taxation treaty". What I meant to say was last time I traded German futures I only paid American taxes as an American living in the United States. So as I understand it the tax does not apply to American traders living in America?
Germany cannot tax citizens from another country that don't live in Germany. In fact no country can do that.
Also just the US can tax its citizens even when they don't live in the country. Only the US and Eritrea does that.
Exactly. That's part of the "American dream". I have friends (European nationality) that went for a 1 year job to the US. Their son was born there three months before they went back to Europe. Today that son is almost 18 years and has to go thru procedures to avoid that he will be taxed as an American. He was less than 3 months of his life in the US and lived whole his life in Europe with a European nationality.