I don't know why people let this happen, but I think it's because of the potential for the situation to get worse or even violent. Some other related thoughts:
1) Selection bias - you've only experienced this with young, black girls. The SDMB taught me the phrase "selection bias."
2) If it seems like a racial thing, it's possibly a socioeconomic class thing. Young people from poor backgrounds may miss a lot of school where habits like waiting your turn are inculcated.
3) It's a youth thing. When people get to a certain age, they realize that they need to follow the rules.
4) And I think the idea that there's a potential for more trouble is tied into expectations about youth, race, and socioeconomic differences.
But yeah, it's annoying as hell.