In short most of German laws make it incredibly risky (but not forbidden) to use any american company for any kind of data that can be resolved to the underlying person. (Eg. a lot of companies got their warning shot when "safe harbor" exploded, if the same happens with https://www.privacyshield.gov/welcome a world of shit awaits)

That said “transferring data” not “American ownership”. If it is required that your data doesn’t leave the country, just use a region in the EU.