Simply put, saying someone can’t wear that band shirt is gatekeeping. Acting hostile towards them because they don’t know much or enough for your standards about the band on their shirt won’t encourage them to get into the music. It makes the person uncomfortable and anxious. This world isn’t a nice place, make it a nicer place and be respectful to other people. It’s just a shirt. People wear sports team shirts all the time and they may not always be sports fans, and who cares?
There are some really important and interesting perspectives in this article. I haven’t worn a band shirt in ages, personally. However, I don’t think it would ever occur to me to do some sort of “purity test” on who is or isn’t allowed to do it.