The simple answer is that the government of Hitler publicly classified Jazz as Jewish music, even though it came from American Blacks. The German news site DW emphasizes the “bravery” of non-Jewish kids in Nazi Germany who “dared to be themselves” by wearing plaid jackets to meet in cafes to keep listening to the forbidden “degenerate” and fremdländisch (alien) tunes.

…not all young people in Nazi Germany supported the regime’s ideology, and for the Swing Youth, jazz music became a vehicle for rebellion. Its members tried to distinguish themselves from Nazi youth movements by appropriating American fashion trends and names. They wore their hair long and dressed in plaid jackets to meet in cafes and clubs playing swing, a jazz sub-genre. They were also said to have greeted one another with the phrase: “Swing Heil!”

The bans on Jazz weren’t strictly enforced, is another way to put it, for those who weren’t Jewish. And the Nazis even remixed Jazz tunes as propaganda.

The Jews of USA have asked Eddie Cantor to write a new version of his famous old-timer “Makin’ Whoopee.” In one of his latest programs on the air, he sang the following song. (Singing) Another war, another profit, another Jewish business trick, another season, another reason for makin’ whoopee. A lot of dough, a lot of gold. The British Empire’s being sold. We’re in the money thanks to Frankie. We’re making whoopee. Washington is our ghetto, Roosevelt our king. Democracy is our motto. Think what a war can bring. We throw our German names away. We are the kikes of USA. You are the goys, folks. We are the boys, folks. We’re making whoopee.

Only towards the end of 1942, as it became clear Germany was going to lose WWII, did Goebbels really clamp down on Germans enjoying Jazz.