I wasn't sure how to react to that. My first thought was do these people not realize the reaction they're naturally going to get?
I mean, sure, the swastika is a very old symbol, predating Nazis by a few thousand years at least, and has usually had positive connotations. But that's not what we're talking about - they're not invoking eastern culture, they're specifically invoking the Nazi connotations, and there isn't anyone who's going to see them wearing it that's not going to associate it with this:
And there is no getting around that association. These people cannot expect people anywhere in the world to look at them wearing Nazi uniforms and not think that these are people who probably hate Jews, Romani, Gays, and the mentally challenged.
And since they say they're not, it begs the question, are these people just wanting to be contrary and stir up trouble because their mothers didn't give them enough attention as children?
I was at an artist gathering where the models were dressed on a steampunk theme a few months ago, and one of the male volunteer models was dressed as an un-dead German soldier, gas mask and everything. He had a character backstory though, that it seemed very important to him that everyone know: This character had been involved in one of the failed assassination attempts on Hitler, and was killed in the attempt. (And subsequently re-animated, but that's beside the point.)
It was important to him that people be reminded that not all Germans in that period were entirely comfortable with Hitler's actions, and there were indeed people who stood up and tried to do something about it. And see, that's a commendable way to present that, and he wasn't being stupid about it. Not only was he trying to educate people, but he recognized that there was no getting around people's reaction to what he was wearing.
So I'm kind of on the fence about it. If it's your thing, I guess, go for it, but don't do it for attention, and don't go around being surprised about how people react to it, and what assumptions will be made about you. And Steampunk and it's derivatives aren't about historical re-enactment anyway. If you just like the style, and you really are into dieselpunk, don't just dress up in a Nazi costume - make it your own some how - use anachronisms, play around with the style, be creative. If it's true dieselpunk, it's about invoking the old images and twisting them, subverting them, turning them upside down, not re-creating them.