I think you are getting a style of dress confused with a personality type. Take me for example, I'm the opposite of emo. Totally emotionless, like a machine. ("happy, pissed off, and don't give a fvck")
But I COULD dye my hair black, part it sloppily over my eye while sticking it up in the back, and get some black frame glasses. Then maybe I get a black t-shirt a size too small, studded belt, highwater jeans. Suddenly I look pretty emo, would you throw tomatoes at me when I walked down the street?
I could also put on a clown suit, or a rockclimber's gear, or a fireman's uniform. All I'm saying it, next time you see a policeman walking down the street, look twice!