Because Mercedes-Benz were Hitler's preferred automobile, even before he took power. Given the very nature of the Third Reich, it was probably wise of them (Mercedes) to pay lip service and not make a fuss about making an obvious stand against Hitler. Like I said in my previous post, they displayed the German flag, which was not uncommon back in the 1930s.
Furthermore, while there were obvious links between the NSKK (
National Socialist Motor Corps) and the NSDAP (
National Socialist German Workers' Party), the former was not under
direct influence of the latter. The NSKK was an SA project; a spinoff of a spinoff. Rudolf Caracciola, for example, was a member of the NSKK as he had to be, but was never a member of the Nazi Party himself.
Don't think for one second that I am trying to justify the fact that the Reichstag contributed state money to both the Auto Unions and Mercedes Grand Prix teams. It happened, but it's similar to saying that all 2011 onwards GMs are Obamamobiles and officially endorse the Affordable Care Act, SOPA and drone strikes.
To play devil's advocate, if you were in charge of the NSDAP or NSKK, wouldn't you use your country's
incredibly successful race teams for propaganda purposes? For the purposes of this Mercedes-Benz, the 540, there is no affirmation in a belief in Nazi ideology.
Also, I do believe this is an Auto Union with a German flag / swastika /
Hakenkreuz on it as well: