Tell that to all of the white nationalist metal bands out there. Not trying to be snarky but metal has had a massive Nazi problem since day 1. Case in point, the metal genre in the OP is riddled with racist scum
Sadly, being open minded doesn't automatically make someone smart. But I've personally had only good experiences with metal fans, so I'm anecdotally primed to believe what you describe is not a very significant problem, or at least not more widespread than in the general population.
More often than not, I meet another metalhead, instant brother or sister. Tend to be cool people.
We moved this summer using an old box truck. Had a breakdown in the middle of Ohio, stuck at a gas station, no cans or uber. Totally told a kid that was gothed up and covered in tats, "Are you a metalhead? Metalheads are cool people, my kid and I are broke down and need a ride." Joey was very cool, gave us a ride to the parts store and a hotel, wouldn't take my money. He likes that Screamo stuff that I'm not real into, we both agree that Maiden, cool jazz, and OTEP are the tits.
Don't get me wrong, Im a metalhead too and generally agree that metalheads are cool people. But you're way more likely to find a Nazi at a metal show than almost any other genre. Punk has a Nazi problem too. It's just the way "outsider" music goes. It attracts everybody on the margins, whether or not they're open minded. That said, if my car was broken down and I needed help from a stranger, I'd still be looking for the metalheads, punks, goths, jughalos, etc. They're generally decent people and it's a shame that we have to deal with shit stains who ruin our collective image
It's definitely a problem. It's up to metalheads to handle it as well. Throw their bitch asses out of shows. Give them shit reviews. Any day is a good day to punch a Nazi.