Why do African Americans champion negative stereotypes?

Recently I moved to a "college town" which is predominately African American. As I understand it, the Philadelphia Department of Corrections has an agreement with the mayor of said town to provide half-way house facilities for paroled inmates in exchange for monetary compensation. Today I witnessed a rather disturbing scene, It was roughly 3:30 pm and I was on my way home from cl when I witnessed four young black children, three male and one female, which appeared to still be in middle school "throwing up gang signs" to a group of four male caucasian college students who were sitting on their porch. I do not know the precursors which lead to these events; I merely witnessed them. My only question is why? Why is that alright? What morals are being instilled in the youth of today? REGARDLESS OF THEIR COLOR. To me, such actions would be perceived as a threat worthy of retaliation. I merely ask why is such hostile behavior overlooked? Also, if your response is a plan to lecture me on social and or racial hierarchy, don't waste your time. Uneducated opinions aren't even worth the second it takes one to formulate them.