Merry Christmas, German friends.
To gain some perspective here, imagine this scenario where Germany was in the US' position and it makes a list of countries: Austria, Belgium, Luxembourg, and Switzerland as nations to exclude from espionage attacks. Imagine that we are in a military alliance with them (NATO) and Germany has military bases spread across the US.
Would that not make you feel uneasy to suddenly learn one day that not only you are a possible military target but also that you are an active espionage target? How can you trust someone who does not trust you and feels the need to spy on everything you do and every word you say?
These are the "Five Eyes" countries and have been closely linked for national intelligence since World War II. The U.S. will monitor literally everyone else that it can, and has done so since WWII proved how useful national security intelligence is for toward national security.
For years you've been able to Google "In God We Trust; All Others We Monitor" and find various intelligence community posters, unit patches, etc.
The leak to SPON helps confirm that with numbers. But Germany's leaders have undoubtedly known it was going on, just as France's, Russia's, etc.