You know better. Those types of headphones have the "DJ look" to them, and I think that is why the style became trendy. I don't think those people really care about quality or actually pretending to be a DJ, rather just having the DJ look.
I have big headphones (not Beats) that I use at home for music production, and I would never wear them outside. I prefer small headphones when I need something on the go, and mine are ultra garbage quality. I think it is ridiculous to carry around a big set of headphones while out and about.
It's not so much the style as it is the celebrity endorsements that attract people to the brand. You got Beats by Dre, Beats DiddyBeats by P Diddy, Beat Heartbeats by Lady Gaga, etc. People also see celebs like Lebron, Katy Perry, Justin Bieber, Lady Gaga, and Kobe wearing them and they want to buy one as well.