Run DMC was inside, NYPD left their motorcycles outside.
1987 was a tough year in NYC. Race relations were poor, the 911 system was stretched to the maximum (computers were being installed) it was an insanely hot summer. The year was so bad NYC EMS issued a medal for just working that year.
This was not the first major rap/hip-hop concert at Madison Square Garden, but it was the first high profile one. White America was just beginning to hear the voice of Black America in their own voice, without interpreters.