Metal culture includes a host of rituals (head banging and moshing), gestures (throwing “devil’s horns” or forming a “claw”) and fashion (battle vests). Importantly, concert attire must adhere to the following unspoken rule: When you see a band play, you should not wear that band’s T-shirt to the show.
Why is wearing the performer’s T-shirt a major etiquette faux pas? After all, sports fans wear their team’s apparel when attending games. Metal shows are different from a sporting event because we assume that everyone at the show is a fan of the headlining (or supporting) band. Wearing a shirt that represents a different band signals your other musical interests to other people at the show. This allows fans to build legitimacy in a tacit way and is a form of word-of-mouth marketing that many bands rely on. Metal T-shirts often showcase incredible artwork and fans want to wear them in a setting where they can be fully appreciated.
The significance of T-shirts to heavy metal extends beyond concert venues. Aside from a momentary trend where death meal shirts posed as high fashion, wearing metal shirts out in the wild serves a broader social function for people who are not exactly known for their social skills. T-shirts are an important tool for network formation in a culture that is largely decentralized and primarily underground. Seeing someone wear a metal shirt that you recognize creates an instant connection and an acknowledgement of participation in a shared community. It often provides the motivation to strike up a conversation. The internet was and continues to be monumental for spreading metal across the globe, but T-shirts are a way for metalheads to find each other in real life.
Can seeing someone wear your favorite band’s shirt inspire a love story?
 The main exception to this rule is the band Ghost. If you attend a Ghost concert, you will observe a sea of people wearing Ghost shirts. I suspect that this is because a significant number of Ghost fans do not have extensive metal interests and thus do not adhere to heavy metal cultural norms. Ghost concerts are still fun, so who cares?
Stay metal, learn economics