Yes, it's because a lot of kids started wearing their dads' classic rock and grunge metal band shirts, so much so that it became fashionable, and stores like Target and Walmart sell these shirts again.
Then you have the kids who actually listen to those bands and think very highly of their learned musical opinions (reminiscent of "le wrong generation" kids).
There's now both a stereotype for the type of trendy person who wears a shirt for a band they've never even heard of and the type of person who judges them as posers.
This isn't new to this generation though, I remember my friend wore a Poison shirt that she bought at Bluenotes when we were like 12 and neither of us even knew it was a band until like a year later when someone gave her shit for wearing it. It was just a cool shirt man, blame capitalism not the kids.
Less than Jake sold a shirt with one of the poison album covers on it but with their name instead. I remember lots of bands doing similar shirts at the time. I had more than a few butt rock lovers try to get a conversation going about how cece demille was so awesome before I even realized what the shirt was mocking.