Trademark is not copyright or patent. Copyright and patents exist to protect the rights of a creator. Trademarks are more about provenance. It protects brands, but also protects consumers of those brands by preventing confusion.

Trademarks are pretty specific. Dr Pepper's trademark on sugary drinks isn't infringed by a company selling black pepper because nobody will confuse the products.

Wal-Mart can sell sodas that are similar to popular drinks like Mountain Dew and Dr Pepper, but they call them something like "Mountain Lightning" and "Dr Thunder". They're clearly meant to be similar products, but they're also clearly not the name-brand stuff. But Walmart can't start selling a knockoff low-calorie soda and call it "Low-Cal Dr Pepper" because it may mislead customers into thinking it's related to Dr Pepper.

And that's essentially what's being claimed here. The band has a trademark on Demon Hunters that specifically covers a band named Demon Hunters selling tickets for live music events. And while there's going to be some understandable confusion given the popularity of the film, the band alleges that the concerts related to the film should be marketed differently. This is especially true when the band from the film already has a different established name.

The commercialization of Christianity this band takes part in upsets me, but they may also be in the right here. They don't want their fans buying tickets to a K-Pop show instead of theirs, and K-Pop fans will also be pretty upset when they buy tickets and go to a show only to get preached at.