S.H.I.E.L.D
There was an ongoing television show named ‘Agents of S.H.I.E.L.D’ that was a spin-off from the Marvel cinematic universe. Fans of the Marvel movies could tune in to the television show to get a sense of where the story was going and what moves they could expect next.
It was very successful, like anything Marvel does, and was a beloved television show by many people worldwide. So it is not surprising that someone would show how much they loved the show by advertising it on their own truck. It has nothing to do with them being a nazi at all.