Why do girls wear band shirts if they don’t listen to the band?

Girls are judged on their appearance, much more than boys. So when a girl gets dressed she has to consider her presentation. Her presentation will make a statement about how she views herself, and how she wants others to view her.

If a girl wears a band shirt she may do it because:
1. She likes the band.
2. It fits her well.
3. It's coordinated with the rest of her outfit.
4. It's fashionable.
5. It has some sentimental value - perhaps someone complimented her about it.

In any case, her presentation is important to her, so say something nice.

One girl who seems generally awesome person to me (but listens to shitty music) wore Metallica t shirt.

Because they like how it fits, and has the added benefit... It looks cool.

Girls are weird man

they're been screwed by some unknown band member

They just like the shirt. Nothing weird about it.

Fashion trends. They are fashion zombies.