I don't believe it's "only" black people. But I do agree that the vast majority(which causes stereotypes) are black, and generally women. I honestly couldn't tell you why anyone does it. Yeah, there are price tags there so you know what you are paying. Sometimes, there isn't a tag, or it's not legible, but the majority of the time that's not the case.
Your question does come off a bit choppy and racist, so I would just forewarn you of that before you get the "REPORT Hammer" knocked on you. You should avoid using words like, "Them" to associate all black people.
In the same token though, people knocking your looks aren't doing any better.